K2 138 Planetary System - Another multi planetary system

 K2-138  Planetary system

K2-138 is notable for its big number of planets, all found through the attempts of citizen scientists. They are named K2-138b, c, d, e, f, and g  in order from their host star. The first five were confirmed by Christiansen et al., while K2-138g wa marked as being a likely candidate. However, since there were only two transits of it, K2-138g could not be confirmed . There is a probability that the two transits for this candidate are from two distinct long-period planets.

All six planets are within the Super-Earth and Mini-Neptune groups , with radius between about 1.6 R⊕ to 3.3 R⊕. The outer five, including the unvalidated K2-138g, are likely small gassy worlds with no solid surface. However, the smaller K2-138b can be rocky. The masses of the planets stay unknown, as the data for K2-138 doesn't have a high enough signal-to-noise ratio for transit-timing variation (TTV) exploration. However, the Spitzer space telescope can be able to accurately find TTVs and lead to the masses of the planets being computed. Planets b through f are predicted to cause TTVs on the order of 2.5 to 7.1 minutes, for predicted masses between 4 M⊕ and 7 M⊕.

The five confirmed planets of K2-138 are very near to the parent star and form an unbroken chain of near-3:2 resonances. Their orbital periods range from 2.35 to 12.76 days, with the unvalidated sixth planet orbiting much further out with a period of about 41 days. K2-138b, c, d, e, and f are locked in various chains of three-body resonances, a feat shared by only a handful of systems, together with TRAPPIST-1 and Kepler-80. Like the former, K2-138 could show the end result of slow, inside disk migration.

Spitzer observations of K2-138g were declared on the AAS Meeting #233. The iPoster shows an updated radius of K2-138g of 3.7 R⊕, making it the biggest planet in the system. This result was initiatory  until being validated in February 2021.

A team of astronomers culled 215 spectra over 79 nights with the instrument HARPS mounted on the ESO 3.6 m Telescope. With a Bayesian analysis of the K2 photometry and HARPS radial-velocities (RVs) the team were able to distrain the mass of planet b to e. The bulk solidities of the planets range from earth-like solidity for planet b to Neptune-like solidity for planet e. The masses and densities constrain the structure of the planets. They have likely rocky cores and a substantial atmospheric sphere, composed of volatiles. For planet f and g this team was able to constrain the upper limitation of the mass to 8.7 and 25.5 earth masses.

K2-138 was selected as a goal by ESA in the first Announcement of Opportunity (AO-1) Program of the CHEOPS mission, which was simulated in December 2019. For 87.6 orbits the spacecraft will record the transits to survey TTVs of the planets. K2-138 can become a benchmark system to compare RV and TTV masses. The system is also a good candidacy to search for co-orbital bodies, which are reckoned to exist and to be stable in resonant bands systems like K2-138.

Host Star

K2-138, also named EPIC 245950175 or EE-1, is a big early K-type main sequence star with a system of up to 6 planets found by citizen scientists. Four were discovered in the first two days of the Exoplanet Explorers project on Zooniverse in early April 2017, while up to two more were published in further exploration. The system is approximately 600 light years away in the constellation Aquarius, within K2 Campaign 12.