# K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System

**Authors:** Kevin K. Hardegree-Ullman, Jessie L. Christiansen, David R. Ciardi, Ian J. M. Crossfield, Courtney D. Dressing, John H. Livingston, Kathryn Volk, Eric Agol, Thomas Barclay, Geert Barentsen, Björn Benneke, Varoujan Gorjian, Martti H. Kristiansen  
**arXiv:** [2102.08978](https://arxiv.org/abs/2102.08978)  
**Date:** 2021-02-17  
**Summary page:** https://www.summarizepaper.com/en/paper/2102.08978v1/

## Key points

- 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.

## Summary

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.