Radiation astronomy/Planets

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This is a multicolor image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. Credit: N. Metcalfe & Pan-STARRS 1 Science Consortium.

"PSO J318.5-22 is a confirmed,[1][2] extrasolar object and candidate planet that does not appear to be orbiting a star. It is approximately 80 light-years away, and belongs to the Beta Pictoris moving group of stars.[3] The object was discovered in 2013 in images taken by the Pan-STARRS PS1 wide-field telescope. The object's age is inferred to be 12 million-years, the same age as the Beta Pictoris group.[4]

“We have never before seen an object free-floating in space that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone".[5]

“Planets found by direct imaging are incredibly hard to study, since they are right next to their much brighter host stars. PSO J318.5-22 is not orbiting a star so it will be much easier for us to study. It is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth”.[6]

Backgrounds[edit]

The near-infrared image shows the GJ 758 solar system. Credit: Max Planck Institute for Astronomy.
This astrometric analysis consists of motions of point-sources near GJ 758 across five epochs (E1–E5), measured relative to GJ 758’s position. Credit: M. Janson et al., National Astronomical Observatory of Japan.

When the overwhelming radiation from the star GJ 758 is reduced and the star itself eclipsed by a disk, secondary radiation sources appear in the background. These are labeled B and C?.

Subsequent observations with the Subaru Telescope revealed C? to be a background star rather than an object in orbit around GJ 758.

"The source tentatively referred to as “GJ 758 C” [follows] the background star track".[7]

"GJ 758 B exhibits common proper motion with its parent star as well as systematic orbital motion towards the northwest, whereas all other point-sources follow the expected trajectory for background stars (solid arrows). The object referred to as “GJ 758 C” [...] is unambiguously identified as a background star (motion highlighted by dashed blue arrows). The grey plus signs are 1σ error bars. The circle marked as “PSF” shows the size of the resolution element in H-band on [High Contrast Instrument for the Subaru Next Generation Adaptive Optics] HiCIAO."[7]

Visuals[edit]

This annotated image shows key features of the Fomalhaut system, including the newly discovered planet Fomalhaut b, and the dust ring. Credit: Credit: NASA, ESA, and Z. Levay (STScI).

The "annotated image [at right] shows key features of the Fomalhaut system, including the newly discovered planet Fomalhaut b, and the dust ring. Also included are a distance scale and an insert, showing how the planet has moved around its parent star over the course of 21 months. The Fomalhaut system is located approximately 25 light-years from the Earth."[8]

The extrasolar planet is in orbit around Fomalhaut and is estimated "to be no more than three times Jupiter's mass ... In 2004, the coronagraph in the High Resolution Camera on Hubble's Advanced Camera for Surveys produced the first-ever resolved visible-light image of a large dust belt surrounding Fomalhaut. It clearly showed that this structure is in fact a ring of protoplanetary debris approximately 21.5 billion miles across with a sharp inner edge. This large debris disk is similar to the Kuiper Belt, which encircles the solar system and contains a range of icy bodies from dust grains to objects the size of dwarf planets, such as Pluto."[9]

"Observations taken 21 months apart by Hubble's Advanced Camera for Surveys' coronagraph show that the object is moving along a path around the star and therefore is gravitationally bound to it. The planet is 10.7 billion miles from the star, or about 10 times the distance of the planet Saturn from the sun."[9]

"A follow-up image in 2006 showed that one of the objects is moving through space with Fomalhaut but changed position relative to the ring since the 2004 exposure. The amount of displacement between the two exposures corresponds to an 872-year-long orbit as calculated from Kepler's laws of planetary motion."[9]

"The planet mysteriously dimmed by half a stellar magnitude between the 2004 and 2006 observations."[9]

Blues[edit]

This image captured by the SOFI instrument on ESO’s New Technology Telescope at the La Silla Observatory shows the free-floating planet CFBDSIR J214947.2-040308.9 in infrared light. Credit: ESO/P. Delorme.

"This image [at the right] captured by the SOFI instrument on ESO’s New Technology Telescope at the La Silla Observatory shows the free-floating planet CFBDSIR J214947.2-040308.9 in infrared light. This object, which appears as a faint blue dot at the centre of the picture and is marked with a cross, is the closest such object to the Solar System. It does not orbit a star and hence does not shine by reflected light; the faint glow it emits can only be detected in infrared light. The object appears blueish in this near-infrared view because much of the light at longer infrared wavelengths is absorbed by methane and other molecules in the planet's atmosphere. In visible light the object is so cool that it would only shine dimly with a deep red colour when seen close-up."[10]

CFBDSIR J214947.2-040308.9 "seems to be part of a nearby stream of young stars known as the AB Doradus Moving Group."[10]

"This is the first isolated planetary mass object ever identified in a moving group, and the association with this group makes it the most interesting free-floating planet candidate identified so far."[10]

“Looking for planets around their stars is akin to studying a firefly sitting one centimetre away from a distant, powerful car headlight. This nearby free-floating object offered the opportunity to study the firefly in detail without the dazzling lights of the car messing everything up.”[10]

"Free-floating objects like CFBDSIR2149 are thought to form either as normal planets that have been booted out of their home systems, or as lone objects like the smallest stars or brown dwarfs. In either case these objects are intriguing — either as planets without stars, or as the tiniest possible objects in a range spanning from the most massive stars to the smallest brown dwarfs."[10]

"The association with the AB Doradus Moving Group would pin down the mass of the planet to approximately 4–7 times the mass of Jupiter, with an effective temperature of approximately 430 degrees Celsius. The planet’s age would be the same as the moving group itself — 50 to 120 million years."[10]

Liquid objects[edit]

This is a detailed, photo-like view of Earth based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Credit: Robert Simmon and Marit Jentoft-Nilsen, NASA.

The image at right is a detailed, photo-like view of Earth based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.

"Viewed from space, the most striking feature of our planet is the water. In both liquid and frozen form, it covers 75% of the Earth’s surface. It fills the sky with clouds. Water is practically everywhere on Earth, from inside the rocky crust to inside our cells."[11]

Mercury[edit]

The color image shown here at right was generated by combining the mosaics taken through the MESSENGER WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

"MESSENGER's Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters. As the spacecraft receded from Mercury after making its closest approach on January 14, 2008, the WAC recorded a 3x3 mosaic covering part of the planet not previously seen by spacecraft. The color image shown here was generated by combining the mosaics taken through the WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). These three images were placed in the red, green, and blue channels, respectively, to create the visualization presented here. The human eye is sensitive only across the wavelength range from about 400 to 700 nanometers. Creating a false-color image in this way accentuates color differences on Mercury's surface that cannot be seen in black-and-white (single-color) images."[12]

"Color differences on Mercury are subtle, but they reveal important information about the nature of the planet's surface material. A number of bright spots with a bluish tinge are visible in this image. These are relatively recent impact craters. Some of the bright craters have bright streaks (called "rays" by planetary scientists) emanating from them. Bright features such as these are caused by the presence of freshly crushed rock material that was excavated and deposited during the highly energetic collision of a meteoroid with Mercury to form an impact crater. The large circular light-colored area in the upper right of the image is the interior of the Caloris basin. Mariner 10 viewed only the eastern (right) portion of this enormous impact basin, under lighting conditions that emphasized shadows and elevation differences rather than brightness and color differences. MESSENGER has revealed that Caloris is filled with smooth plains that are brighter than the surrounding terrain, hinting at a compositional contrast between these geologic units. The interior of Caloris also harbors several unusual dark-rimmed craters, which are visible in this image. The MESSENGER science team is working with the 11-color images in order to gain a better understanding of what minerals are present in these rocks of Mercury's crust."[12]

Venus[edit]

This real-color image of Venus is processed from imaging through the clear and blue filters on Mariner 10. Credit: NASA or Ricardo Nunes.

In the image at right the whiter cloud areas have a bluish tint. This is a real-color image of Venus processed through the clear and blue filters onboard Mariner 10.

Mars[edit]

These are true color images of Mars taken in 1999. Credit: Antonio Cidadao.
These are Hubble Space Telescope images of Mars prior to the Mars Pathfinder spacecraft and Lander. Credit: Philip James, NASA.

"The [true] color images of Mars [at right] were taken in 1999, across almost 60 million miles (!) by a talented amateur astronomer in Oeiras, Portugal – Antonio Cidadao."[13]

"They were acquired with a modest 10-inch "Schmidt-Cassegrain" reflecting telescope, and a commercially available CCD (charge coupled device) camera. Mr. Cidadao’s total investment in his "Mars imaging system"—commercial telescope and electronic camera, plus computer to process the images, and the appropriate software—was approximately three thousand American dollars."[13]

"In 1997, before the arrival of the Mars Pathfinder spacecraft (the first NASA Lander sent to Mars since Viking), the Hubble Telescope was tasked to acquire a series of "weather forecast Mars images" prior to the landing [at left]."[13]

"This long-distance reconnaissance detected a small dust storm less than a month before the Pathfinder arrival, which (with its potentially high winds) could have posed a serious threat to the Pathfinder entry and landing."[13]

"If dust diffuses to the landing site, the sky could turn out to be pink like that seen by Viking... otherwise [based on the Hubble images - above], Pathfinder will likely show blue sky with bright clouds."[14]

Jupiter[edit]

Cloud bands are clearly visible on Jupiter. Credit: NASA/JPL/USGS.

"[O]range [is] the color of Jupiter".[15]

The orange and brown coloration in the clouds of Jupiter are caused by upwelling compounds that change color when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are believed to be phosphorus, sulfur or possibly hydrocarbons.[16][17] These colorful compounds, known as chromophores, mix with the warmer, lower deck of clouds. The zones are formed when rising convection cells form crystallizing ammonia that masks out these lower clouds from view.[18]

Saturn[edit]

The view of Saturn from Hubble, taken on March 22, 2004, is so sharp that many individual Saturnian ringlets can be seen. Credit: NASA, ESA and Erich Karkoschka (University of Arizona).

"The view [at right] from Hubble [of Saturn], taken on March 22, 2004, is so sharp that many individual Saturnian ringlets can be seen."[19]

"Hubble's exquisite optics, coupled with the high resolution of its Advanced Camera for Surveys, allow it to take pictures of Saturn which are nearly as sharp as Cassini's, even though Hubble is nearly a billion miles farther from Saturn than Cassini."[19]

"Camera exposures in four filters (blue, blue-green, green, and red) were combined to form the Hubble image, to render colors similar to what the eye would see through a telescope focused on Saturn. The subtle pastel colors of ammonia-methane clouds trace a variety of atmospheric dynamics. Saturn displays its familiar banded structure, and haze and clouds of various altitudes. Like Jupiter, all bands are parallel to Saturn's equator. Even the magnificent rings, at nearly their maximum tilt toward Earth, show subtle hues, which indicate the trace chemical differences in their icy composition."[19]

Uranus[edit]

This is an image of the planet Uranus taken by the spacecraft Voyager 2 in 1986. Credit: NASA/JPL/Voyager mission.
Uranus's southern hemisphere in approximate natural colour (left) and in shorter wavelengths (right), shows its faint cloud bands and atmospheric "hood" as seen by Voyager 2. Credit: NASA.
The first dark spot on Uranus ever observed is in an image obtained by ACS on HST in 2006. Credit: NASA, ESA, L. Sromovsky and P. Fry (University of Wisconsin), H. Hammel (Space Science Institute), and K. Rages (SETI Institute).
Uranus in 2005. Rings, southern collar and a bright cloud in the northern hemisphere are visible (HST ACS image).

In larger amateur telescopes with an objective diameter of between 15 and 23 cm, the planet appears as a pale cyan disk with distinct limb darkening.

"Methane possesses prominent absorption bands in the visible and near-infrared (IR) making Uranus aquamarine or cyan in color."[20]

In 1986 Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).[21] Their boundary is located at about -45 degrees of latitude. A narrow band straddling the latitudinal range from -45 to -50 degrees is the brightest large feature on the visible surface of the planet.[21][22] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above).[23] Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[21] In all other respects Uranus looked like a dynamically dead planet in 1986. Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor a polar cap in the northern hemisphere.[22] So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[22] In 2007, when Uranus passed its equinox, the southern collar almost disappeared, while a faint northern collar emerged near 45 degrees of latitude.[24]

On August 23, 2006, researchers at the Space Science Institute (Boulder, CO) and the University of Wisconsin observed a dark spot on Uranus's surface, giving astronomers more insight into the planet's atmospheric activity.[25] Why this sudden upsurge in activity should be occurring is not fully known, but it appears that Uranus's extreme axial tilt results in extreme seasonal variations in its weather.[26][27] Determining the nature of this seasonal variation is difficult because good data on Uranus's atmosphere have existed for less than 84 years, or one full Uranian year. A number of discoveries have been made. Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes.[28] A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.[29] Stratospheric temperature measurements beginning in the 1970s also showed maximum values near the 1986 solstice.[30] The majority of this variability is believed to occur owing to changes in the viewing geometry.[31]

There are some reasons to believe that physical seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.[27] During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.[28] This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.[27] Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.[27] Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),[23] while the northern hemisphere demonstrated increasing activity,[32] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.[33] This indeed happened in 2007 when the planet passed an equinox: a faint northern polar collar arose, while the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.[24]

Neptune[edit]

This picture from the Voyager 2 sequence shows two of the four cloud features which have been tracked by the Voyager cameras during the past two months. Credit: NASA.

A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,[34] although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.[35]

Haumea[edit]

Hubble Space Telescope image of Haumea (center) and its two moons; Hiʻiaka is above Haumea and Namaka is below. Credit: Renerpho.{{free media}}
Haumea's orbit outside of Neptune is similar to Makemake's. The positions are as of 1 January 2018. Credit: Tomruen.{{free media}}

Haumea (minor-planet designation 136108 Haumea, initially, (136108) 2003 EL61) is a dwarf planet located beyond Neptune's orbit.[36] It was discovered on December 28, 2004, just after Christmas,[37] at the Palomar Observatory.[38] Precovery images of Haumea have been identified back to March 22, 1955.[39]

Haumea is a plutoid, a dwarf planet located beyond Neptune's orbit.[40] The nominal trajectory suggests that Haumea is in a weak 7:12 orbital resonance with Neptune, which would make it a resonant trans-Neptunian object instead.[41] There are precovery images of Haumea dating back to March 22, 1955 from the Palomar Mountain Digitized Sky Survey.[42]

Haumea has an orbital period of 284 Earth years, a perihelion of 35 AU, and an orbital inclination of 28°.[39] It passed aphelion in early 1992,[43] and is currently more than 50 AU from the Sun.[44]

Haumea's orbit has a slightly greater orbital eccentricity than that of the other members of the Haumea family, its collisional family. This is thought to be due to Haumea's weak 7:12 orbital resonance with Neptune gradually modifying its initial orbit over the course of a billion years,[45][46] through the Kozai mechanism, or Kozai effect, which allows the exchange of an orbit's inclination for increased eccentricity.[45][47][48]

With a visual magnitude of 17.3,[44] Haumea is the third-brightest object in the Kuiper belt after Pluto and Makemake, and easily observable with a large amateur telescope.[49] However, because the planets and most small Solar System bodies share a invariable plane, or common orbital alignment, from their formation in the protoplanetary, primordial disk, of the Solar System, most early surveys for distant objects focused on the projection on the sky of this common plane, called the ecliptic.[50] As the region of sky close to the ecliptic became well explored, later sky surveys began looking for objects that had been dynamically excited into orbits with higher inclinations, as well as more distant objects, with slower mean motions across the sky.[51][52]

Haumea displays large fluctuations in brightness over a period of 3.9 hours, which can only be explained by a rotational period of this length.[53] This is faster than any other known equilibrium body in the Solar System, and indeed faster than any other known body larger than 100 km in diameter.[49] While most rotating bodies in equilibrium are flattened into oblate spheroids, Haumea rotates so quickly that it is distorted into a triaxial ellipsoid. If Haumea were to rotate much more rapidly, it would distort itself into a dumbbell shape and split in two.[36] This rapid rotation is thought to have been caused by the impact that created its satellites and collisional family.[45] Because Haumea has moons, the mass of the system can be calculated from their orbits using Kepler's third law. The result is 4.2×1021 kg, 28% the mass of the Plutonian system and 6% that of the Moon. Nearly all of this mass is in Haumea.[54][55]

For most distant objects, the albedo is unknown, but Haumea is large and bright enough for its infrared, thermal emission to be measured, which has given an approximate value for its albedo and thus its size.[56]

The rigid body dynamics, specifically, rotational physics of deformable bodies predicts that over as little as a hundred days,[49] a body rotating as rapidly as Haumea will have been distorted into the hydrostatic equilibrium form of a triaxial ellipsoid. It is thought that most of the fluctuation in Haumea's brightness is caused not by local differences in albedo but by the alternation of the side view and end view as seen from Earth.[49]

If Haumea were in hydrostatic equilibrium and had a low density like Pluto, with a thick mantle of volatiles, such as ice, over a small silicate, rocky core, its rapid rotation would have elongated it to a greater extent than the fluctuations in its brightness allow. Such considerations constrained its density to a range of 2.6–3.3 g/cm3.[57][49]

In 2005, the Gemini Observatory and Keck Observatory telescopes obtained spectra of Haumea which showed strong crystalline water ice features similar to the surface of Pluto's moon Charon.[58] This is peculiar, because crystalline ice forms at temperatures above 110 K, whereas Haumea's surface temperature is below 50 K, a temperature at which amorphous ice is formed.[58] In addition, the structure of crystalline ice is unstable under the constant rain of cosmic rays and energetic particles from the Sun that strike trans-Neptunian objects.[58] The timescale for the crystalline ice to revert to amorphous ice under this bombardment is on the order of ten million years,[59] yet trans-Neptunian objects have been in their present cold-temperature locations for timescales of billions of years.[46] Radiation damage should also redden and darken the surface of trans-Neptunian objects where the common surface materials of organic molecular ices and tholin-like compounds are present, as is the case with Pluto. Therefore, the spectra and colour suggest Haumea and its family members have undergone recent resurfacing that produced fresh ice. However, no plausible resurfacing mechanism has been suggested.[60]

Haumea is as bright as snow, with an albedo in the range of 0.6–0.8, consistent with crystalline ice.[49] Other large TNOs such as Eris appear to have albedos as high or higher.[61] Best-fit modeling of the surface spectra suggested that 66% to 80% of the Haumean surface appears to be pure crystalline water ice, with one contributor to the high albedo possibly hydrogen cyanide or phyllosilicate clays.[58] Inorganic cyanide salts such as copper potassium cyanide may also be present.[58]

Visible and near infrared spectra suggest a homogeneous surface covered by an intimate 1:1 mixture of amorphous and crystalline ice, together with no more than 8% organics. The absence of ammonia hydrate excludes cryovolcanism and the observations confirm that the collisional event must have happened more than 100 million years ago, in agreement with the dynamic studies.[62] The absence of measurable methane in the spectra of Haumea is consistent with a warm collisional history that would have removed such volatiles,[58] in contrast to Makemake.[63]

50000 Quaoar[edit]

Quaoar is imaged by the Hubble Space Telescope in 2002. Credit: NASA and M. Brown (Caltech).{{free media}}
Hubble photo is used to measure size of Quaoar. Credit: NASA.{{free media}}
Polar and ecliptic view of Quaoar's orbit compared to Pluto and various other cubewanos. Quaoar's orbit is colored yellow in the left image Credit: Eurocommuter and blue in the right image Credit: kheider.

50000 Quaoar, provisional designation 2002 LM60, is a non-resonant trans-Neptunian object (classical Kuiper belt object, or cubewano) and a possible dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune measuring approximately 1,100 km (680 mi) in diameter, about half the diameter of Pluto, discovered at the Palomar Observatory on 6 June 2002.[64] Signs of water ice on the surface of Quaoar have been found, which suggests that cryovolcanism may be occurring on Quaoar.[65] A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects.[66] In February 2007, Weywot, a synchronous minor-planet moon in orbit around Quaoar, was discovered by Brown.[67] Weywot is measured to be 80 km (50 mi) across. Both objects were named after mythological figures from the Native American Tongva people in Southern California. Quaoar is the Tongva creator deity and Weywot is his son.[68]

The earliest precovery, or prediscovery image, of Quaoar was found on a photographic plate imaged on 25 May 1954 from the Palomar Observatory Sky Survey.[69]

Quaoar's albedo or reflectivity could be as low as 0.1, which would still be much higher than the lower estimate of 0.04 for 20000 Varuna. This may indicate that fresh ice has disappeared from Quaoar's surface.[70] The surface is moderately red, meaning that Quaoar is relatively more reflective in the red and near-infrared spectrum than in the blue.[71] The Kuiper belt objects Varuna and Ixion are also moderately red in the spectral class. Larger Kuiper belt objects are often much brighter because they are covered in more fresh ice and have a higher albedo, and thus they present a neutral color.[72] A 2006 model of internal heating via radioactive decay suggested that, unlike 90482 Orcus, Quaoar may not be capable of sustaining an internal ocean of liquid water at the mantle–core boundary.[73]

The presence of methane and other volatiles on Quaoar's surface suggest that it may support a tenuous atmosphere produced from the sublimation of volatiles.[74] With a measured mean temperature of ~ 44 K (−229.2 °C), the upper limit of Quaoar's atmospheric pressure is expected to be in the range of a few microbars.[74] Due to Quaoar's small size and mass, the possibility of Quaoar having an atmosphere of nitrogen and carbon monoxide has been ruled out, since the gases would escape from Quaoar.[74] The possibility of a methane atmosphere still remains, with the upper limit being less than 1 microbar.[75][74] In 2013, Quaoar occulted a 15.8 magnitude star and revealed no sign of a substantial atmosphere, placing an upper limit to at least 20 nanobars, under the assumption that Quaoar's mean temperature is 42 K (−231.2 °C) and that its atmosphere consists of mostly methane.[75][74]

Quaoar is thought to be an oblate spheroid around 1,100 km (680 mi) in diameter, being slightly flattened in shape.[75] The estimates come from observations of Quaoar as it occulted a 15.8 magnitude star in 2013.[75] Given that Quaoar has an estimated oblateness value of 0.0897±0.006 and a measured equatorial diameter of 1,138+48
−34
 km
, Quaoar is believed to be in hydrostatic equilibrium, being described as a Maclaurin spheroid.[75] Quaoar is about as large and massive as (if somewhat smaller than) Pluto's moon Charon.[a] Quaoar is roughly half the size of Pluto.[78]

TW Hydrae[edit]

These images, taken a year apart by NASA's Hubble Space Telescope, reveal a shadow moving counterclockwise around a gas-and-dust disk encircling the young star TW Hydrae. Credit: NASA, ESA and J. Debes (STScl).

"TW Hydrae, a star 176 light-years from Earth in the constellation Hydra ... which has about the same mass as the sun, is surrounded by a dense ring of gas and dust. ... Its circumstellar disk is estimated to between 3 million and 10 million years old, and most protoplanetary disks are thought to last only 2 million to 3 million years. ... [Using the] ESA's Herschel Space Telescope, which is sensitive to the required infrared wavelengths [to measure the amount of deuterium] ... The ratio of deuterium to hydrogen appears constant in Earth's region of space, which means ... measuring hydrogen deuteride [yields] how much regular molecular hydrogen is present. ... TW Hydrae's disk is at least 16,650 times the mass of the Earth. Considering the planets in the solar system may have arisen from a disk only as little as 3,300 times the mass of the Earth, the matter in TW Hydrae's disk would be ample to form a planetary system. "This ... seems to point towards different systems finding disparate pathways to making planets. ... Signs of hydrogen deuteride remain difficult to detect around distant stars".[79]

"These images [on the right], taken a year apart by NASA's Hubble Space Telescope, reveal a shadow moving counterclockwise around a gas-and-dust disk encircling the young star TW Hydrae. The two images at the top show an uneven brightness across the disk. Through enhanced image processing (images at the bottom), the darkening becomes even more apparent."[80]

The "shadow [was noticed] after analyzing 18 years' worth of observations of TW Hydrae, which is about 8 million years old and lies 192 light-years from Earth, in the constellation Hydra. The images, taken by NASA's Hubble Space Telescope, showed that the shadow rotates around the 41-billion-mile-wide (66 billion kilometers) disk once every 16 years."[81]

"This is the very first disk where we have so many images over such a long period of time, therefore allowing us to see this interesting effect. That gives us hope that this shadow phenomenon may be fairly common in young stellar systems."[80]

"An unseen exoplanet is the best explanation for the shadow."[80]

The "alien world itself isn't casting the shadow; rather, the planet's gravity has twisted and tilted the inner portion of the dust-and-gas disk, blocking starlight headed toward the outer reaches."[81]

The "planet lies about 100 million miles (160 million km) from TW Hydrae — about the distance from Earth to the sun. That's way too close to the star for Hubble or any other telescope now in operation to photograph it directly. (Planets that orbit so tightly are drowned out by their parent stars' overwhelming glare.)"[81]

"The putative planet must be about five times more massive than Jupiter to sculpt the inner disk in this manner."[80]

"What is surprising is that we can learn something about an unseen part of the disk by studying the disk's outer region and by measuring the motion, location and behavior of a shadow. This study shows us that even these large disks, whose inner regions are unobservable, are still dynamic, or changing in detectable ways which we didn't imagine."[80]

CHXR 73 b[edit]

CHXR 73 b is a star which lies at the border between planet and brown dwarf. Credit: NASA/ESA/K. Luhman (Penn State University, USA).

"Astronomers using the NASA/ESA Hubble Space Telescope have photographed one of the smallest objects ever seen around a normal star beyond our Sun. Weighing in at 12 times the mass of Jupiter, the object is small enough to be a planet. The conundrum is that it's also large enough to be a brown dwarf, a failed star."[82]

"New, more sensitive telescopes are finding smaller and smaller objects of planetary-mass size. These discoveries have prompted astronomers to ask the question, are planetary-mass companions always planets?"[82]

"The object is so far away from its star that it is unlikely to have formed in a circumstellar disk."[82]

"The study of substellar objects in orbit around a star allows us to determine the age, and over time also the mass of the companion. Such studies help us to improve out understanding of the formation and inner structure of brown dwarfs and planets."[83]

GJ 504 b[edit]

These are near-infrared color composite images of a “second Jupiter” around the Sun-like star GJ 504. Credit: M. Kuzuhara, et al., National Astronomical Observatory of Japan.

"Exoplanets are planets orbiting stars other than our Sun, outside of our Solar System. As of July 2013, most of the 890 exoplanets reported thus far have been discovered by indirect observation techniques, e.g. monitoring the host star for radial velocity variation or planetary transits [Indirect observations detect exoplanets by noting the central star's velocity shift (radial velocity method) or the dimming of stellar light as the planet passes by (transit method).]."[84]

"Such techniques require observations over at least one orbital period and are impractical for detecting planets that are widely separated from their host stars and have long orbital periods."[84]

Direct "imaging may be the most important way to observe exoplanets, because it yields information about the planet's luminosity, temperature, atmosphere, and orbit."[84]

"Astronomers have recently discovered and captured an image of the least massive planet ever imaged so far -- a so-called "second Jupiter". This discovery marks an important step toward the direct imaging of much fainter Earth-like planets in the future and may lead to new models of planet formation."[84]

"Near-infrared color composite images of a “second Jupiter” around the Sun-like star GJ 504 [are at the right]. A coronagraph and differential techniques suppress the bright light from the central star. On the left is the intensity image, which shows the radiant power passing through the area, while on the right is the signal-to-noise ratio image, which shows the weakest signal that the detecting system can recognize."[84]

"Based on the relation of its observed luminosity and estimated age in comparison with the theoretical model, scientists can infer that GJ 504 b has a mass as small as three Jovian masses. If so, it is the lightest-mass planet ever imaged. The apparent distance between the central star and planet is 44 AU (astronomical unit), which is larger than Neptune's orbit and comparable to Pluto's".[84]

Kepler-1520 b[edit]

The planetary system of KIC 12557548 consists of one extrasolar planet, named Kepler-1520 b which appears to possess a tail of dust and gas formed in a similar fashion to that of a comet[85] but, as opposed to the tail of a comet, it contains molecules of pyroxene and aluminium(III) oxide. Based on the rate at which the particles in the tail are emitted, the mass of the planet has been constrained to less than 0.02 Earth masses — a higher-mass planet would have too much gravity to sustain the observed rate of mass loss.[86] [87]

Kepler-1520 is at J2000 19h 23m 51.8899s[88] +51° 30′ 16.98″[88] in the constellation Cygnus, with an apparent magnitude in the visible of 16.7,[87] spectral class K4V[89] details

  1. mass = 0.76 ± 0.03[90]
  2. radius = 0.71 ± 0.026[90]
  3. luminosity = 0.14[87]
  4. surface gravity (log g) = 4.610 +0.018 or −0.031 cgs[90]
  5. temperature = 4677 +82 or -71[90]
  6. metallicity/Fe = 0.04 ± 0.15[90]
  7. rotation = 22.91±0.24 days[91]
  8. age in gyr = 4.47[90]

The star is particularly important, as measurements taken by the Kepler spacecraft indicate that the variations in the star's light curve cover a range from about 0.2% to 1.3% of the star's light being blocked.[87] This indicates that there may be a rapidly disintegrating planet, a prediction not yet conclusively confirmed, in orbit around the star, losing mass at a rate of 1 Earth mass every billion years.[87] The planet itself is about 0.1 Earth masses,[92] or just twice the mass of Mercury, and is expected to disintegrate in about 100[92]-200 million years.[87] The planet orbits its star in just 15.7 hours,[87] at a distance only two stellar diameters away from the star's surface,[85] and has an estimated effective temperature of about 2255 K.[92] The orbital period of the planet is one of the shortest ever detected in the history of the extrasolar planet search.[93] In 2016, the planet was confirmed as part of a data release by the Kepler spacecraft.

K2-33 b[edit]

The planet is best known for its remarkably young age, which is estimated to be about 9.3 million years.[94]

Given this age, the planetary system most likely formed back near the end of the Miocene epoch of the Earth's history. Observations made on the planet confirmed that it was in fact a fully formed exoplanet, not just a protoplanet that was still in the stages of developing. The mass and radius of the exoplanet further help constrain this statement.[94][95]

  1. semimajor = 0.0409 +0.0021 or −0.0023 AU[96]
  2. eccentricity = 0.0[94]
  3. orbital period = 5.424865 +0.000035 −0.000031[2] d[94]
  4. inclination = 89.1 +0.6 or −1.1[94]
  5. mass = 3.6 MJ.[95]
  6. mean radius = 5.04 +0.34 or −0.37 R[94]

K2-33b is the youngest confirmed transiting exoplanet.

The discoveries of K2-33b and V830 Tau b are most notable for explaining how close-in planets form, an open question in the field of exoplanets since the discovery of the first exoplanet, 51 Pegasi b, in 1995. Given the young ages of these exoplanets, several theories of planetary migration can be ruled out because they take too long to form close-in planets. The most plausible formation scenario for K2-33b is that it formed further away from its star, then migrated inwards through the protoplanetary disk, although it remains a possibility that the planet formed in place.[94][95]

"The question we are answering is: Did those planets take a long time to get into those hot orbits, or could they have been there from a very early stage? We are saying, at least in this one case, that they can indeed be there at a very early stage."[97]

NASA's Kepler spacecraft began its "Second Light" mission from 23 August to 13 November 2014, collecting data from the core of Upper Scorpius, which included K2-33.[94] The exoplanet was simultaneously discovered by two independent research groups.[95][94]

The star K2-33 was studied on days in late January, February, and March 2016. The observations were made with the Immersion Grating Infrared Spectrometer (IGRINS) on the 2.7-m Harlan J. Smith Telescope at the McDonald Observatory.[94] After observing the respective transits, which for K2-33b occurred roughly every 5 days (its orbital period), it was eventually concluded that a planetary body was responsible for the periodic 5-day transits.[97]

HD 95086 b[edit]

This image from ESO's Very Large Telescope (VLT) shows the newly discovered planet HD95086 b, next to its parent star. Credit: ESO/J. Rameau.{{free media}}

"This image from ESO's Very Large Telescope (VLT) shows the newly discovered planet HD95086 b, next to its parent star. The observations were made using NACO, the adaptative optics instrument for the VLT in infrared light, and using a technique called differential imaging, which improves the contrast between the planet and its dazzling host star. The star itself has been removed from the picture during processing to enhance the view of the faint exoplanet and its position is marked. The exoplanet appears at the lower left."[98]

"The blue circle is the size of the orbit of Neptune in the Solar System."[98]

"The star HD 95086 has similar properties to Beta Pictoris and HR 8799 around which giant planets have previously been imaged at separations between 8 and 68 astronomical units. These stars are all young, more massive than the Sun, and surrounded by a debris disc."[98]

HIP 78530 b[edit]

Between 2000 and 2001, the ADONIS: ADaptive Optics Near Infrared System system at the ESO 3.6 m Telescope in Chile detected a faint object in the vicinity of HIP 78530, reported in 2005 and 2007.[99]

A random selection of ninety-one stars in the Upper Scorpius association provided a sample of stars to be observed using the Near Infrared Imager and Spectrometer (NIRI) and Altitude conjugate Adaptive Optics for the Infrared (ALTAIR) adaptive optics system at the Gemini Observatory for direct imaging including HIP 78530, first imaged by the camera on May 24, 2008.[99] This initial image revealed the presence of the same faint object within the vicinity of HIP 78530.[99]

Follow-up imaging took place on July 2, 2009 and August 30, 2010 using the same instruments, with additional follow-up data recovered in the spring and summer of 2010, with the data used to filter out pixelated portions of the images and improve the images' quality to suggest that the faint object in the image was near the star HIP 78530, was a brown dwarf or planet.[99]

HIP 78530 b is most likely a brown dwarf, a massive object that is large enough to fuse deuterium but not large enough to ignite and become a star, but HIP 78530 b's characteristics blend the line between whether or not it is a brown dwarf or a planet.[99]

  1. semimajor axis = 710 (± 60)[100] AU
  2. orbital period = ~12000[99] y
  3. mass = 23.04 (± 4)[100] MJ
  4. surface gravity = -2.55 (± 0.13)[99] g
  5. temperature = 2800 (± 200)[99] K

HIP 78530 is a bright, blue B-type main sequence star in the Upper Scorpius association, a loose star cluster composed of stars with a common origin.[99] The star is estimated to be approximately 2.5 times the mass of the Sun, with an age of the Upper Scorpius group of approximately 11 million years old.[101]

Its effective temperature is estimated at 10,500 K[100] less than twice the effective temperature of the Sun.[102]

HIP 78530 has an apparent magnitude of 7.18.[100] It is incredibly faint, if visible at all, as seen from the unaided eye of an observer on Earth.[103]

HR 8799[edit]

This image shows the light from three planets orbiting HR 8799 (indicated with an 'X') 120 light-years away. Credit: NASA/JPL-Caltech/Palomar Observatory.

The image at the right "shows the light from three planets orbiting a star 120 light-years away. The planets' star, called HR8799, is located at the spot marked with an "X.""[104]

"This picture was taken using a small, 1.5-meter (4.9-foot) portion of the Palomar Observatory's Hale Telescope, north of San Diego, Calif. This is the first time a picture of planets beyond our solar system has been captured using a telescope with a modest-sized mirror -- previous images were taken using larger telescopes."[104]

"The three planets, called HR8799b, c and d, are thought to be gas giants like Jupiter, but more massive. They orbit their host star at roughly 24, 38 and 68 times the distance between our Earth and sun, respectively (Jupiter resides at about 5 times the Earth-sun distance)."[104]

2M1207b[edit]

This composite image shows an exoplanet (the red spot on the lower left), orbiting the brown dwarf 2M1207 (centre). Credit: ESO.

"This composite image [at the right] shows an exoplanet (the red spot on the lower left), orbiting the brown dwarf 2M1207 (centre). 2M1207b is the first exoplanet directly imaged and the first discovered orbiting a brown dwarf. It was imaged the first time by the VLT in 2004. Its planetary identity and characteristics were confirmed after one year of observations in 2005. 2M1207b is a Jupiter-like planet, 5 times more massive than Jupiter. It orbits the brown dwarf at a distance 55 times larger than the Earth to the Sun, nearly twice as far as Neptune is from the Sun. The system 2M1207 lies at a distance of 230 light-years, in the constellation of Hydra. The photo is based on three near-infrared exposures (in the H [1.66 μm], K [2.18 μm] and L [3.8 μm] wavebands) with the NACO adaptive-optics facility at the 8.2-m VLT Yepun telescope at the ESO Paranal Observatory."[105]

Teegarden's Star b and c[edit]

Generalised Lomb-Scargle (GLS) periodogram shows near-infrared (NIR) radial velocities (RVs) compared to visual (VIS) RVs. Credit: M. Zechmeister, et al..{{fairuse}}

"Teegarden’s Star was discovered in this century by Teegarden et al. (2003). It is the 24th nearest star to the Sun7 with a distance of 3.831 pc. The spectral type is M7.0 V (Alonso-Floriano et al. 2015), making it the brightest representative of this and later spectral classes with a J magnitude of 8.39 mag (V = 15.08 mag). Schweitzer et al. (2019) derived the effective temperature Teff, metallicity [Fe/H], and surface gravity log g from fitting PHOENIX synthetic spectra (Husser et al. 2013) to CARMENES spectra following the method of Passegger et al. (2018). They obtained the luminosity L with Gaia DR2 parallax and integrated broad-band photometry as described in Cifuentes et al. (in prep. 2019). Schweitzer et al. (2019) then estimated the stellar radius R via Stefan-Boltzmann’s law and finally a stellar mass M of 0.089 M by using their own linear mass-radius relation."[106]

Toff = 2904 ± 51 K (Schweitzer et al. (2019)), or 2637 ± 30 K ( Rojas-Ayala et al. (2012)).[106]

P [d] = 4.9100 ± 0.0014 for Teegarden’s Star planet b and 11.409 ± 0.009 for planet c.[106] "Both orbits are circular within the eccentricity uncertainties. The eccentricity posteriors cumulate near zero and have a one-sided distribution."[106]

"The minimum masses are mb sinI = 1.05 M and mc sini = 1.11 M. To estimate true masses, we further drew for each sample an inclination from a uniform distribution of cos i, which corresponds to geometrically random orientations (Kürster et al. 2008). The median values of the true masses are around 16 % higher than the minimum masses (cosI = 0.5 → 1/ sinI = 1.155)."[106]

V830 Tau b[edit]

The hot Jupiter exoplanet V830 Tau b is the youngest known exoplanet with an age of around 2 million years (around the time that humans evolved on Earth).[107]

4 Vesta[edit]

As NASA's Dawn spacecraft takes off for its next destination, this mosaic synthesizes some of the best views the spacecraft had of the giant asteroid Vesta. Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA.
Location and structure of the Vesta family are depicted. Credit: Deuar.

In the full image of Vesta at right, the rocky-object appears to have suffered from meteor damage.

Vesta, minor-planet designation 4 Vesta, is one of the largest asteroids in the Solar System. It lost some 1% of its mass less than a billion years ago in a collision that left an enormous crater occupying much of its southern hemisphere. Debris from this event has fallen to Earth as howardite–eucrite–diogenite (HED) meteorites, a rich source of information about the asteroid.[108][109]

"V-type asteroids are bodies whose surfaces are constituted of basalt. In the Main Asteroid Belt, most of these asteroids are assumed to come from the basaltic crust of Asteroid (4) Vesta."[110]

The Vestian asteroids consist "of 4 Vesta, the second-most-massive of all asteroids (mean diameter of 530 km), and many small asteroids below 10 km diameter. The brightest of these, 1929 Kollaa and 2045 Peking, have an absolute magnitude of 12.2, which would give them a radius of about 7.5 km assuming the same high albedo as 4 Vesta."[111]

"A HCM numerical analysis (by Zappala 1995) determined a large group of 'core' family members, whose proper orbital elements lie in the approximate ranges"[111]

ap ep ip
min 2.26 AU 0.075 5.6°
max 2.48 AU 0.122 7.9°

"This gives the approximate boundaries of the family. At the present epoch, the range of  osculating orbital elements of these core members is"[111]

a e i
min 2.26 AU 0.035 5.0°
max 2.48 AU 0.162 8.3°

"The Zappala 1995 analysis[112] found 235 core members. A search of a recent proper-element database (AstDys) for 96944 minor planets in 2005 yielded 6051 objects (about 6% of the total) lying within the Vesta-family region as per the first table above."[111]

Technology[edit]

The North telescope of the Gemini Observatory directly imaged the HIP 78530 system. Credit: Mailseth.
Gemini South is on Cerro Pachón in Chile. Credit: Denys.{{free media}}
Comparison is of nominal sizes of primary mirrors of notable optical telescopes. Credit: Cmglee.{{free media}}
Gemini Planet Imager (GPI) image is of a planet orbiting a distant star known as 51 Eridani. Credits: J. Rameau (Univ. of Montreal) and C. Marois (NRC Herzberg, Canada).

In the lower image on the right, the bright central star 51 Eridani has been mostly removed by a hardware and software mask to enable the detection of the exoplanet (labelled "b") that is one millionth as bright.

Both Gemini telescopes employ sophisticated state-of-the-art adaptive optics systems: Gemini-N routinely uses the ALTAIR system, built in Canada, which achieves a 30%-45% Strehl ratio on a 22.5-arcsecond-square field and can feed NIRI, NIFS or GNIRS;[113] it can use natural or laser guide stars. In conjunction with NIRI it was responsible for the discovery of HR 8799 b.

At Gemini-S the Gemini Multi-Conjugate Adaptive Optics System (GeMS) may be used with the FLAMINGOS-2 near-infrared imager and spectrometry, or the Gemini South Adaptive Optics Imager (GSAOI), which provides uniform, diffraction-limited image quality to arcminute-scale fields of view, GeMS achieved first light on December 16, 2011.[114] Using a constellation of five laser guide stars, it achieved Full width at half maximum (FWHM) of 0.08 arc-seconds in the H band over a field of 87 arc-seconds square.

The detectors in each instrument have recently been upgraded with Hamamatsu Photonics devices, which significantly improve performance in the far red part of the optical spectrum (700–1,000 nm).[115]

Near-infrared imaging and spectroscopy are provided by the NIRI, NIFS, GNIRS, FLAMINGOS-2, and GSAOI instruments.[116]

The Gemini Planet Imager (GPI) is a high contrast imaging instrument that was built for the Gemini South Telescope that achieves high contrast at small angular separations, allowing for the direct imaging and integral field spectroscopy of extrasolar planets around nearby stars.[117][118]

See also[edit]

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