Catalog of New K2 Exoplanet Candidates from Citizen Scientists

TL;DR

This paper presents a catalog of 28 new exoplanet candidates from K2 data, identified through citizen science efforts, including a notable multi-planet system, enhancing understanding of exoplanet diversity and multiplicity.

Contribution

The study introduces a new catalog of 28 K2 exoplanet candidates vetted by citizen scientists, including the discovery of a multi-planet system with near-resonant sub-Neptune planets.

Findings

28 new exoplanet candidates identified

Discovery of a multi-planet system in near 3:2 resonance

Candidates have RV amplitudes within current detection capabilities

Abstract

We provide 28 new planet candidates that have been vetted by citizen scientists and expert astronomers. This catalog contains 9 likely rocky candidates ($R_{pl} < 2.0R_\oplus$) and 19 gaseous candidates ($R_{pl} > 2.0R_\oplus$). Within this list we find one multi-planet system (EPIC 246042088). These two sub-Neptune ($2.99 \pm 0.02R_\oplus$ and $3.44 \pm 0.02R_\oplus$) planets exist in a near 3:2 orbital resonance. The discovery of this multi-planet system is important in its addition to the list of known multi-planet systems within the K2 catalog, and more broadly in understanding the multiplicity distribution of the exoplanet population (Zink et al. 2019). The candidates on this list are anticipated to generate RV amplitudes of 0.2-18 m/s, many within the range accessible to current facilities.

Full text

Catalog of New K2 Exoplanet Candidates from Citizen Scientists

Jon K. Zink

Department of Physics and Astronomy, University of California, Los Angeles, CA 90095

Caltech/IPAC-NASA Exoplanet Science Institute, Pasadena, CA 91125

Kevin K. Hardegree-Ullman

Caltech/IPAC-NASA Exoplanet Science Institute, Pasadena, CA 91125

Jessie L. Christiansen

Caltech/IPAC-NASA Exoplanet Science Institute, Pasadena, CA 91125

Ian J. M. Crossfield

Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139

Erik A. Petigura

Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA

Sagan Fellow

Chris J. Lintott

Oxford Astrophysics, The Denys Wilkinson Building, Oxford OX1 3RH, United Kingdom

John H. Livingston

Department of Astronomy, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

David R. Ciardi

Caltech/IPAC-NASA Exoplanet Science Institute, Pasadena, CA 91125

Geert Barentsen

Bay Area Environmental Research, Moffett Field, CA 94035

Courtney D. Dressing

Department of Astronomy, University of California, Berkeley 94720

Alexander Ye

Department of Astronomy, University of California, Berkeley 94720

Joshua E. Schlieder

Exoplanets and Stellar Astrophysics Laboratory, Code 667, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Kevin Acres

Exoplanet Explorers, Citizen Scientist

Peter Ansorge

Exoplanet Explorers, Citizen Scientist

Dario Arienti

Exoplanet Explorers, Citizen Scientist

Elisabeth Baeten

Exoplanet Explorers, Citizen Scientist

Victoriano Canales Cerdá

Exoplanet Explorers, Citizen Scientist

Itayi Chitsiga

Exoplanet Explorers, Citizen Scientist

Maxwell Daly

Exoplanet Explorers, Citizen Scientist

James Damboiu

Exoplanet Explorers, Citizen Scientist

Martin Ende

Exoplanet Explorers, Citizen Scientist

Adnan Erdag

Exoplanet Explorers, Citizen Scientist

Stiliyan Evstatiev

Exoplanet Explorers, Citizen Scientist

Joseph Henderson

Exoplanet Explorers, Citizen Scientist

David Hine

Exoplanet Explorers, Citizen Scientist

Tony Hoffman

Exoplanet Explorers, Citizen Scientist

Emmanuel Lambrou

Exoplanet Explorers, Citizen Scientist

Gabriel Murawski

Exoplanet Explorers, Citizen Scientist

Mark Nicholson

Exoplanet Explorers, Citizen Scientist

Mason Russell

Exoplanet Explorers, Citizen Scientist

Hans Martin Schwengeler

Exoplanet Explorers, Citizen Scientist

Alton Spencer

Exoplanet Explorers, Citizen Scientist

Aaron Tagliabue

Exoplanet Explorers, Citizen Scientist

Christopher Tanner

Exoplanet Explorers, Citizen Scientist

Melina Thévenot

Exoplanet Explorers, Citizen Scientist

Christine Unsworth

Exoplanet Explorers, Citizen Scientist

Jouni Uusi-Simola

Exoplanet Explorers, Citizen Scientist

planets and satellites: detection

https://www.overleaf.com/23750108cfjmqhyykmmr

1 Exoplanet Explorers

The K2 mission has successfully found $\approx\!1,000$ new exoplanet candidates.111https://exoplanetarchive.ipac.caltech.edu Now with an enormous data set ($\approx~{}400,000$ stellar targets) that nearly doubles the source count of Kepler (Huber et al., 2016), data parsing provides a unique time intensive obstacle. The Exoplanet Explorers222https://www.zooniverse.org/projects/ianc2/exoplanet-explorers project, part of the Zooniverse platform, allows citizen scientists to help overcome the abundance of transit data (Christiansen et al., 2018). We make available 204,855 statistically significant dips in K2 light curves from campaigns 0-8, 10, and 12-14. We used the k2phot pipeline (Petigura et al., 2018) to remove the K2 systematics and searched for periodic transits using the TERRA search algorithm (Petigura et al., 2013). For training, each participant is shown an example of a real folded exoplanet transit light curve, with the expected model plotted over the data. The volunteer is then instructed to look for dips that provide a similar match to this basic transit model. Each folded light curve presented are assigned a “Yes” or “No” value by the citizen scientist, indicating their belief that the source of the dip is caused by a transiting exoplanet. This simple visual inspection helps create a targeted search of the K2 light curves.

2 Criteria for Selection

For selection within this catalog we require additional cuts to provide a more focused list of potential candidates. We only consider dips that have received a “Yes” vote by $\geq 90\%$ of the reviewers. All sources have been reviewed by at least 20 citizen scientists. Furthermore, we remove planet candidates that have been previously noted on the ExoFOP333https://exofop.ipac.caltech.edu/k2/ website (as of February 20, 2019). To eliminate eclipsing binaries, we exclude candidates with an inferred planet radius $R_{\rm pl}$ $>0.2R_{\sun}$. An expert visual inspection of the fitted light curve is performed to further eliminate noisy and problematic fits. Strong V-shape transits have been removed. These grazing transits only provide a lower limit for the planetary radius, making it difficult to eliminate eclipsing binaries.

3 Fit Method

Utilizing the EMCEE algorithm (Foreman-Mackey et al., 2013), we maximize and sample the posterior of the 8 transit parameters. The fitted parameters are as follows: $R_{\rm pl}/R_{\star}$ (the radius ratio of the planet and star), planet period, $T_{0}$ (the center of the first detected transit), $a_{\rm pl}/R_{\star}$ (the ratio of the semi-major axis to stellar radius), $b$ (the impact parameter), two quadratic limb darkening parameters, and a floating flux normalization parameter. Each parameter is fit using a uniform prior, with the exception of the semi-major axis ratio ($a_{\rm pl}/R_{\star}$) and the two limb darkening parameters. Assuming a perfect initial measurement of period from the from the TERRA grid search, Kepler’s law is used to derive a Gaussian prior for $a_{\rm pl}/R_{\star}$. The independent information of stellar radius and stellar mass are provided by Huber et al. (2016) with Gaia radius updates from Bailer-Jones et al. (2018). Since the uncertainty in mass and radius are non-negligible, this constraint is rather weak. The priors for the limb darkening parameters are calculated using a Monte Carlo interpolation of the appropriate Claret et al. (2012) table.

We use the BATMAN transit model (Kreidberg, 2015) to fit the processed K2Phot light curves. Here a Gaussian likelihood function is implemented to fit the photometric data to the transit model. This transit fitting procedure is similar to that of Crossfield et al. (2016). The resulting parameters are provided in our supplementary blog.

4 Results

In Figure 1 we provide 28 new planet candidates that have been vetted by citizen scientists and expert astronomers. This catalog contains 9 likely rocky candidates ($R_{\rm pl}\leq 2.0R_{\earth}$) and 19 gaseous candidates ($R_{\rm pl}>2.0R_{\earth}$). Within this list we find one multi-planet system (EPIC 246042088). These two sub-Neptune ($2.99\pm 0.02R_{\earth}$ and $3.44\pm 0.02R_{\earth}$) planets exist in a near 3:2 orbital resonance. The discovery of this multi-planet system is important in its addition to the list of known multi-planet systems within the K2 catalog, and more broadly in understanding the multiplicity distribution of the exoplanet population (Zink et al., 2019). The candidates on this list are anticipated to generate RV amplitudes of 0.2-18 m/s, many within the range accessible to current facilities.

Acknowledgement

We thank all of the 21,770 volunteers who have helped classify transits via Exoplanet Explorers. The following users were unable to be contacted, but also provided early classifications: Cleaver82, DarylW, EEdiscoverer, garryway, GeorgeHolbrook, Grayzer56, Ianbourns, jgraber, krbethune, miguelgambler, mwalden, mzslashx, Or2dee2, sona25, swiese, Toncent, and willedwards45. This publication uses data generated via the Zooniverse.org platform, development of which is funded by generous support, including a Global Impact Award from Google, and by a grant from the Alfred P. Sloan Foundation. This research has made use of the Exoplanet Follow-up Observation Program website and 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.