Editing Exoplanet (section)

=== Color and brightness ===
{{See also|Sudarsky's gas giant classification}}The apparent brightness ([[apparent magnitude]]) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with a high albedo that is far from the star.<ref>[http://phl.upr.edu/library/notes/theapparentbrightnessandsizeofexoplanetsandtheirstars The Apparent Brightness and Size of Exoplanets and their Stars] {{Webarchive|url=https://web.archive.org/web/20140812200814/http://phl.upr.edu/library/notes/theapparentbrightnessandsizeofexoplanetsandtheirstars|date=12 August 2014}}, Abel Mendez, updated 30 June 2012, 12:10&nbsp;pm</ref>[[File:Color HD 189733b vs solar system.jpg|thumb|upright=1.6|alt=Color-color diagram comparing the colors of Solar System planets to exoplanet HD 189733b. HD 189733b reflects as much green as Mars and almost as much blue as Earth.|This [[color–color diagram]] compares the colors of planets in the Solar System to exoplanet [[HD 189733b]]. The exoplanet's deep blue color is produced by [[silicate]] droplets, which scatter blue light in its atmosphere.]]
In 2013, the color of an exoplanet was determined for the first time. The best-fit [[albedo]] measurements of [[HD 189733b]] suggest that it is deep dark blue.<ref>{{Cite web|last=Garner|first=Rob|date=2016-10-31|title=NASA Hubble Finds a True Blue Planet|url=http://www.nasa.gov/content/nasa-hubble-finds-a-true-blue-planet|access-date=2022-01-17|website=NASA}}</ref><ref>{{Cite journal | doi = 10.1088/2041-8205/772/2/L16|arxiv=1307.3239| title = The Deep Blue Color of HD189733b: Albedo Measurements with Hubble Space Telescope/Space Telescope Imaging Spectrograph at Visible Wavelengths| journal = The Astrophysical Journal| volume = 772| issue = 2| pages = L16| year = 2013| last1 = Evans | first1 = T. M. | last2 = Pont | first2 = F. D. R. | last3 = Sing | first3 = D. K. | last4 = Aigrain | first4 = S.|author-link4=Suzanne Aigrain | last5 = Barstow | first5 = J. K. | last6 = Désert | first6 = J. M. | last7 = Gibson | first7 = N. | last8 = Heng | first8 = K. | last9 = Knutson | first9 = H. A. | last10 = Lecavelier Des Etangs | first10 = A. |bibcode=2013ApJ...772L..16E|s2cid=38344760}}</ref> Later that same year, the colors of several other exoplanets were determined, including [[GJ 504 b]] which visually has a magenta color,<ref name="Kuzuhara2013">{{Cite journal|arxiv=1307.2886|title=Direct Imaging of a Cold Jovian Exoplanet in Orbit around the Sun-like Star GJ 504|journal=The Astrophysical Journal|volume=774|issue=11|page=11|date=2013|display-authors=etal|doi=10.1088/0004-637X/774/1/11|bibcode = 2013ApJ...774...11K |last1=Kuzuhara|first1=M.|last2=Tamura|first2=M.|last3=Kudo|first3=T.|last4=Janson|first4=M.|last5=Kandori|first5=R.|last6=Brandt|first6=T. D.|last7=Thalmann|first7=C.|last8=Spiegel|first8=D.|last9=Biller|first9=B.|last10=Carson|first10=J.|last11=Hori|first11=Y.|last12=Suzuki|first12=R. |last13=Burrows |first13=Adam |last14=Henning|first14=T.|last15=Turner|first15=E. L.|last16=McElwain|first16=M. W.|last17=Moro-Martín|first17=A.|last18=Suenaga|first18=T.|last19=Takahashi|first19=Y. H.|last20=Kwon|first20=J.|last21=Lucas|first21=P.|last22=Abe|first22=L.|last23=Brandner|first23=W.|last24=Egner|first24=S.|last25=Feldt|first25=M.|last26=Fujiwara|first26=H.|last27=Goto|first27=M.|last28=Grady|first28=C. A.|last29=Guyon|first29=O.|last30=Hashimoto|first30=J.<!-- the rest: Y. Hayano, M. Hayashi, S. S. Hayashi, K. W. Hodapp, M. Ishii, M. Iye, G. R. Knapp, T. Matsuo, S. Mayama, S. Miyama, J.-I. Morino, J. Nishikawa, T. Nishimura, T. Kotani, N. Kusakabe, T. -S. Pyo, E. Serabyn, H. Suto, M. Takami, N. Takato, H. Terada, D. Tomono, M. Watanabe, J. P. Wisniewski, T. Yamada, H. Takami, T. Usuda -->|s2cid=53343537|url=https://pure.uva.nl/ws/files/2002826/150064_Direct_Imaging_of_a_Cold_Jovian_Exoplanet.pdf}}</ref> and [[Kappa Andromedae b]], which if seen up close would appear reddish in color.<ref name="arXiv1211.3744">{{cite journal|title=Direct Imaging Discovery of a 'Super-Jupiter' Around the late B-Type Star Kappa And|date=15 November 2012|arxiv=1211.3744|author1=Carson|author2=Thalmann|author3=Janson|author4=Kozakis|author5=Bonnefoy|author6=Biller|author7=Schlieder|author8=Currie|author9=McElwain|bibcode = 2013ApJ...763L..32C |doi = 10.1088/2041-8205/763/2/L32|volume=763|issue=2|journal=The Astrophysical Journal|pages=L32|s2cid=119253577}}</ref> [[Helium planet]]s are expected to be white or grey in appearance.<ref name=SpaceDaily-2015-06-16>{{cite news |url= http://www.spacedaily.com/reports/Helium_Shrouded_Planets_May_Be_Common_in_Our_Galaxy_999.html |title= Helium-Shrouded Planets May Be Common in Our Galaxy |publisher= SpaceDaily |date= 16 June 2015 |access-date=3 August 2015}}</ref>

The darkest known planet in terms of [[geometric albedo]] is [[TrES-2b]], a [[hot Jupiter]] that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres, but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.<ref name=darkest_news>{{cite web |url=http://www.space.com/12612-alien-planet-darkest-coal-black-kepler.html |title=Coal-Black Alien Planet Is Darkest Ever Seen |date=11 August 2011 |publisher=Space.com |access-date=12 August 2011}}</ref><ref>{{cite journal |arxiv=1108.2297 |bibcode=2011MNRAS.417L..88K |doi=10.1111/j.1745-3933.2011.01127.x |volume=417 |issue=1 |title=Detection of visible light from the darkest world |journal=Monthly Notices of the Royal Astronomical Society: Letters |pages=L88–L92 |year=2011 |last1=Kipping |first1=David M. |last2=Spiegel |first2=David S. |doi-access=free |s2cid=119287494}}</ref><ref>{{Cite journal |doi=10.1088/0004-637X/761/1/53 |arxiv=1210.4592 |title=Photometrically derived masses and radii of the planet and star in the TrES-2 system |journal=The Astrophysical Journal |volume=761 |issue=1 |page=53 |year=2012 |last1=Barclay |first1=T. |last2=Huber |first2=D. |last3=Rowe |first3=J. F. |last4=Fortney |first4=J. J. |last5=Morley |first5=C. V. |last6=Quintana |first6=E. V. |last7=Fabrycky |first7=D. C. |last8=Barentsen |first8=G. |last9=Bloemen |first9=S. |last10=Christiansen |first10=J. L. |last11=Demory |first11=B. O. |last12=Fulton |first12=B. J. |last13=Jenkins |first13=J. M. |last14=Mullally |first14=F. |last15=Ragozzine |first15=D. |last16=Seader |first16=S. E. |last17=Shporer |first17=A. |last18=Tenenbaum |first18=P. |last19=Thompson |first19=S. E. |bibcode=2012ApJ...761...53B |s2cid=18216065}}</ref>

For [[gas giant]]s, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.<ref name="wfirstreturn">{{cite arXiv|eprint=1412.6097 |last1=Burrows |first1=Adam |title=Scientific Return of Coronagraphic Exoplanet Imaging and Spectroscopy Using WFIRST |class=astro-ph.EP |year=2014 }}</ref>

There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully [[Planetary phase|phase]]-dependent, this is not always the case in the near infrared.<ref name="wfirstreturn" />

Temperatures of gas giants reduce over time and with distance from their stars. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures, ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.<ref name="wfirstreturn" />