Results of the summarizing process for the arXiv paper: 2102.08978v1

The $K2$ mission has greatly expanded our ability to discover new exoplanets, with citizen scientists helping to identify transiting planets with orbital periods below 40 days through the Exoplanet Explorers project. This led to the discovery of one super-Earth and four sub-Neptune sized planets in the relatively bright ($V=12.21$, $K=10.3$) K2-138 system, all of which orbit near 3:2 mean motion resonances. The $K2$ light curve showed two additional transit events consistent with a sixth planet, which was later validated by astronomers using $Spitzer$ photometry and measured its radius to be $3.44^{+0.32}_{-0.31}\,R_{\oplus}$. This solidifies K2-138 as the $K2$ system with the most currently known planets, bringing the total number of confirmed planets in this system to six - named K2-138 g - with an orbital period of 41.966 days and orbits within a sizable gap between the outer two known planets in the system - K2-138 e and K2-138 f - which have orbital periods of 12.76 and 31.6 days respectively. Due to its relative brightness and near resonance of its inner planets, K2-138 could be a key benchmark system for both radial velocity and transit timing variation mass measurements; indeed radial velocity masses for the inner four planets have already been obtained. With five sub-Neptunes and one super-Earth, it provides a unique test bed for comparative atmospheric studies of warm to temperate planets of similar size, dynamical studies of near resonant planets, and models of planet formation and migration. This discovery highlights not only the power of citizen science in discovering new exoplanets but also emphasizes the importance of follow up observations with other telescopes to confirm and characterize these planets.

• Citizen scientists helped identify transiting planets with orbital periods below 40 days through the Exoplanet Explorers project
• One super-Earth and four sub-Neptune sized planets were discovered in the K2-138 system, all orbiting near 3:2 mean motion resonances
• Two additional transit events consistent with a sixth planet were observed and later validated by astronomers using Spitzer photometry
• K2-138 is now the K2 system with the most currently known planets, bringing the total number of confirmed planets in this system to six (named K2-138 g)
• Due to its relative brightness and near resonance of its inner planets, K2-138 could be a key benchmark system for both radial velocity and transit timing variation mass measurements
• With five sub-Neptunes and one super-Earth, it provides a unique test bed for comparative atmospheric studies of warm to temperate planets of similar size, dynamical studies of near resonant planets, and models of planet formation and migration
• This discovery highlights not only the power of citizen science in discovering new exoplanets but also emphasizes the importance of follow up observations with other telescopes to confirm and characterize these planets.

The K2 Mission and the Discovery of the K2-138 System

The Kepler mission, launched in 2009, revolutionized our understanding of exoplanets. Its successor, the $K2$ mission, has continued this legacy by greatly expanding our ability to discover new exoplanets. Citizen scientists have been instrumental in identifying transiting planets with orbital periods below 40 days through the Exoplanet Explorers project. This led to the discovery of one super-Earth and four sub-Neptune sized planets in the relatively bright ($V=12.21$, $K=10.3$) K2-138 system, all of which orbit near 3:2 mean motion resonances.

Confirmation and Characterization of a Sixth Planet

The $K2$ light curve showed two additional transit events consistent with a sixth planet, which was later validated by astronomers using $Spitzer$ photometry and measured its radius to be $3.44^{+0.32}_{-0.31}\,R_{\oplus}$. This solidifies K2-138 as the only known system discovered by $K2$ that contains six confirmed planets - named K2-138 g - with an orbital period of 41.966 days and orbits within a sizable gap between the outer two known planets in the system - K2-138 e and K2-138 f - which have orbital periods of 12.76 and 31.6 days respectively.

Implications for Future Research

Due to its relative brightness and near resonance of its inner planets, K2-138 could be a key benchmark system for both radial velocity and transit timing variation mass measurements; indeed radial velocity masses for the inner four planets have already been obtained With five sub-Neptunes and one super Earth it provides a unique test bed for comparative atmospheric studies of warm to temperate planets of similar size , dynamical studies of near resonant planets ,and models of planet formation migration .This discovery highlights not only power citizen science discovering new exoplanets but also emphasizes importance follow up observations other telescopes confirm characterize these planets .