A Caltech Library Service

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

Hardegree-Ullman, Kevin K. and Christiansen, Jessie L. and Ciardi, David R. and Crossfield, Ian J. M. and Dressing, Courtney D. and Livingston, John H. and Volk, Kathryn and Agol, Eric and Barclay, Thomas and Barentsen, Geert and Benneke, Björn and Gorjian, Varoujan and Kristiansen, Martti H. (2021) K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System. Astronomical Journal, 161 (5). Art. No. 219. ISSN 0004-6256. doi:10.3847/1538-3881/abeab0.

[img] PDF - Published Version
See Usage Policy.

[img] PDF - Accepted Version
Creative Commons Attribution.


Use this Persistent URL to link to this item:


K2 greatly extended Kepler's ability to find new planets, but it was typically limited to identifying transiting planets with orbital periods below 40 days. While analyzing K2 data through the Exoplanet Explorers project, citizen scientists helped discover one super-Earth and four sub-Neptune sized planets in the relatively bright (V = 12.21, K = 10.3) K2-138 system, all which orbit near 3:2 mean-motion resonances. The K2 light curve showed two additional transit events consistent with a sixth planet. Using Spitzer photometry, we validate the sixth planet's orbital period of 41.966 ± 0.006 days and measure a radius of 3.44_(-0.31)^(+0.32) R_⊕, solidifying K2-138 as the K2 system with the most currently known planets. There is a sizeable gap between the outer two planets, since the fifth planet in the system, K2-138 f, orbits at 12.76 days. We explore the possibility of additional nontransiting planets in the gap between f and g. Due to the relative brightness of the K2-138 host star, and the near resonance of the inner planets, K2-138 could be a key benchmark system for both radial velocity and transit-timing variation mass measurements, and indeed radial velocity masses for the inner four planets have already been obtained. With its five sub-Neptunes and one super-Earth, the K2-138 system 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Hardegree-Ullman, Kevin K.0000-0003-3702-0382
Christiansen, Jessie L.0000-0002-8035-4778
Ciardi, David R.0000-0002-5741-3047
Crossfield, Ian J. M.0000-0002-1835-1891
Dressing, Courtney D.0000-0001-8189-0233
Livingston, John H.0000-0002-4881-3620
Volk, Kathryn0000-0001-8736-236X
Agol, Eric0000-0002-0802-9145
Barclay, Thomas0000-0001-7139-2724
Barentsen, Geert0000-0002-3306-3484
Benneke, Björn0000-0001-5578-1498
Gorjian, Varoujan0000-0002-8990-2101
Kristiansen, Martti H.0000-0002-2607-138X
Additional Information:© 2021. The American Astronomical Society. Received 2020 September 11; revised 2021 February 12; accepted 2021 February 16; published 2021 April 8. We would like to thank Matt Russo and SYSTEM Sounds for their creative sonification of the K2-138 system: K.K.H.U. would like to thank Jon Zink for helpful discussions regarding transit fitting. This work is based in part on observations made with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This paper includes data collected by the K2 mission. Funding for the K2 mission is provided by the NASA Science Mission directorate. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This research made use of Astropy, 27 a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013, 2018). This research made use of exoplanet (Foreman-Mackey et al. 2020) and its dependencies (Astropy Collaboration et al. 2013, 2018; Kipping 2013; Foreman-Mackey et al. 2017, 2020; Salvatier et al. 2016; Theano Development Team 2016; Foreman-Mackey 2018; Van Eylen et al. 2019; Luger et al. 2019; Agol et al. 2020). K.V. acknowledges funding from NASA (grant 80NSSC18K0397). Facilities: Spitzer - Spitzer Space Telescope satellite, Kepler - , Exoplanet Archive. - Software: Astropy (Astropy Collaboration et al. 2013, 2018), batman (Kreidberg 2015), dustmaps (Green et al. 2018), DYNAMITE (Dietrich & Apai 2020), emcee (Foreman-Mackey et al. 2013), exoplanet (Foreman-Mackey et al. 2020), isoclassify (Huber et al. 2017), NumPy (Harris et al. 2020), photutils (Bradley et al. 2019), SciPy (Virtanen et al. 2020), TTVFaster (Agol & Deck 2016).
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:Exoplanet systems; Exoplanets; Mini Neptunes; Super Earths
Issue or Number:5
Classification Code:Unified Astronomy Thesaurus concepts: Exoplanet systems (484); Exoplanets (498); Mini Neptunes (1063); Super Earths (1655)
Record Number:CaltechAUTHORS:20210423-164903172
Persistent URL:
Official Citation:Kevin K. Hardegree-Ullman et al 2021 AJ 161 219
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:108841
Deposited By: George Porter
Deposited On:29 Apr 2021 14:51
Last Modified:16 Nov 2021 19:32

Repository Staff Only: item control page