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Kepler-22 Exoplanet System

First ever transiting exoplanet discovered in the habitable zone //

Kepler-22, also known as KOI-87, is a real star located 635 light-years away in the constellation Cygnus. As a yellow dwarf of spectral type G5V, it is slightly smaller and dimmer than the Sun's G2V classification but more advanced in age at an estimated 7 billion years old. Kepler-22 comes in at 0.857 solar masses and 0.869 solar radii with a 5596 K (5323 °C) surface temperature. It was among the first stars to be studied by the Kepler space telescope[en.wikipedia.org], just days after commencing its science operations back in 2009.

Kepler-22 is the parent to one known exoplanet—Kepler-22 b. When its existence was confirmed in 2011, it became the first-ever transiting exoplanet discovered in the habitable zone of another star. Kepler-22 b follows a 290-day orbit at an average distance of 0.812 AU. Assuming that its path is roughly circular, it would fit comfortably within the inner edge of Kepler-22's habitable zone and receive almost the same amount of energy as Earth does from the Sun. In order to obtain a basic assessment of its habitability, astronomers use a measure called the planetary equilibrium temperature, which describes the approximate temperature a planet would have if it were only heated by its parent star. Kepler-22 b is predicted to have an equilibrium temperature of 279 K (6 °C)—slightly higher than Earth's 255 K (-18 °C). Its actual temperature is of course subject to various other factors like the greenhouse effect and any internal heating.

In terms of physical properties, Kepler-22 b is 2.1 times the Earth's size and up to 9.1 times in mass, so it would be a stretch to describe it as a close parallel to our own. In fact, it is unlikely to be rocky at all and is most probably abundant in volatiles like a marine world or mini-Neptune. Basic modelling has suggested that the planet could have a water mass fraction of 56.9%, corresponding to an enormous ocean extending down thousands of kilometres with a mantle of exotic, highly pressurised ice.

As widely known, planets can become tidally locked to their star if they orbit too closely, bringing a host of caveats for their habitability and climatical stability. Unlike most exoplanets discovered so far, Kepler-22 b's fairly long 290-day orbit means it probably isn't tidally locked at this point in time. Assuming that it began with an initial rotation period of 1 day, models suggest that it would spin once every ~2–3 Earth days for the system's current age. Complete tidal locking of the planet is predicted to occur around 10 billion years at the earliest, though by that point, its parent star may well be nearing the end of its life. Naturally, if Kepler-22 b has a large moon as Earth does, that would dramatically change the course of its tidal evolution. However, the existence of any exomoons greater than 0.54 Earth masses has been ruled out.

This simulation replicates the Kepler-22 system from real data wherever possible, though fictional elements may be incorporated due to knowledge gaps:

• I’ve imagined Kepler-22 b as a temperate superoceanic super-Aquaria with a cyan atmosphere over deep greenish waters. Its appearance is based on the artist impression[exoplanets.nasa.gov] in NASA's media release. This illustration does not depict a thick gaseous envelope, but the possibility remains open that the real exoplanet resembles more a mini-Neptune than an ocean world.
  
• In this simulation, a day on Kepler-22 b lasts three Earth days, with fairly mild "seasons" over its year due to a smaller axial tilt. Two ice caps at its north and south are the only solid surfaces amid the endless ocean, both of which are surrounded by powerful stormy vortices that blow the frozen landscapes into a near-circular expanse. The planet has an average temperature of 312 K (39 °C), rising to 324 K (51 °C) at the equator and dropping to 233 K (-40 °C) at the poles.
  
• The true mass of Kepler-22 b is unknown; only an upper mass limit is cited. This upper limit has been refined over the years and currently stands at 9.1 Earth masses, which is chosen for this simulation.
  
• The eccentricity of Kepler-22 b's orbit, or how “circular” it is, is also not well-constrained and is only known to be less than 0.72. Thus, the planet could still have a very oval path that just partly lies in the habitable zone. For simplicity, this simulation assumes a perfectly circular orbit with an eccentricity of 0.

A special mention to @...v.2 for requesting this system.



GSC 03546-02301, KIC 10593626, SPOCS 3045, UCAC4 690-066662, AP J19165219+4753040, KOI-87, TIC 158984573, Gaia DR3 2127941757262806656, Kepler-22, 2MASS J19165219+4753040, UCAC3 276-148830, Gaia DR2 2127941757262806656

• Adibekyan, V., Figueira, P. and Santos, N.C., 2017. How alien can alien worlds be?. arXiv preprint arXiv:1710.07482.
  
• Barnes, R., 2017. Tidal locking of habitable exoplanets. Celestial Mechanics and Dynamical Astronomy, 129, pp.509-536.
  
• Bonomo, A.S., Dumusque, X., Massa, A., Mortier, A., Bongiolatti, R., Malavolta, L., Sozzetti, A., Buchhave, L.A., Damasso, M., Haywood, R.D. and Morbidelli, A., 2023. Cold Jupiters and improved masses in 38 Kepler and K2 small planet systems from 3661 HARPS-N radial velocities-No excess of cold Jupiters in small planet systems. Astronomy & Astrophysics, 677, p.A33.
  
• Borucki, W.J., Koch, D.G., Batalha, N., Bryson, S.T., Rowe, J., Fressin, F., Torres, G., Caldwell, D.A., Christensen-Dalsgaard, J., Cochran, W.D. and DeVore, E., 2012. Kepler-22b: a 2.4 Earth-radius planet in the habitable zone of a Sun-like star. The Astrophysical Journal, 745(2), p.120.
  
• Kipping, D.M., Forgan, D., Hartman, J., Nesvorný, D., Bakos, G.A., Schmitt, A. and Buchhave, L., 2013. The hunt for exomoons with Kepler (HEK). III. The first search for an exomoon around a habitable-zone planet. The Astrophysical Journal, 777(2), p.134.