Exoplanet Explorers Discoveries – 28 New K2 Planet Candidates | Zooniverse

Hello Exoplanet Explorers! As we begin to wrap up the K2 mission, we must now analyze the data provided by our wonderful citizen scientists. Through your work in classifying these stellar light curves, we have been able to identify 28 previously unknown exoplanet candidates. This list has been published as a Research Note of the American Astronomical Society (RNAAS) and provides targets for future observations. These candidates all present a weak transiting signal and were missed by previous automated searches of the K2 data set. With the help of citizen scientists on the Exoplanet Explorers project, these transits were clearly spotted. Each candidate was flagged as a transiting planet by at least 90% of the classifiers who reviewed the light curve, and was subsequently verified as a planet-like signal by an expert scientist.

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These new exoplanet candidates range in size from ~2/3 the radius of Earth to nearly twice the radius of Neptune. As seen in the figure above, most of these planets are similar in orbital period and planet radius to the bulk population of K2 confirmed and candidate planets. However, this list provides 9 potentially rocky planets with radii less than twice that of Earth which is where we think planets transition from being primarily rocky like Earth and have a thick gaseous atmosphere like Neptune. The radius cut off for rocky planets remains somewhat unclear as the composition of the planet is the important feature to consider here. Unfortunately, transiting planets only provide us with the planets radius. Nevertheless, optimistic estimates suggest that planets smaller than twice the radius of Earth may be rocky. For further reading on this topic check out the associated paper! As the search continues for habitable planets, these rocky planets are of unique interest as they potentially mimic the environment we experience here on Earth. The planets in our list are outside the habitable zone, but there are some candidates that are close. We need better stellar properties, which might indeed show that some planets could be in the habitable zone.

"In 2017, citizen scientists on Exoplanet Explorers helped discover 4 planets in the K2-138 systems. K2-138 is now known to host at least 6 planets, with 5 of them in a near-perfect 3:2 orbital resonance chain. The new Exoplanet Explorers candidate EPIC 246042088 appears to contain two gaseous planets also in a 3:2 orbital resonance, with periods of 11.82 and 8.04 days. These multiple planet systems continue to be important in our understanding of planet formation and the occurrence of planet multiplicity.

It appears that at least 1 in 4 stars exists in a multiple star system (for the smallest stars), and that fraction only increases for higher mass stars. That means it is likely that many of our candidates could be in a multiple star system, which would drastically change the properties of the planets. This is because a multiple star system will produce a more light and cause us to under-estimate the planets radius. It is crucial that follow up observations are made to examine the host stars of these systems. The Exoplanet Explorers team also just announced the discovery of K2-228Bb. This unique planet is part of a low mass binary star system and appears to orbit the within the habitable zone of its host star. For more information on this discovery check out the associated paper!

To aid future observations, we provide all of the derived planet properties in the following table. This table motivates future observations that will measure the mass of these candidates and can provide enough evidence to confirm that the dips were caused by transiting planets (rather than eclipsing binaries). Once mass measurements have been made, we can determine the density of these planets and begin to learn about the planet's compositions.

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We thank all 21,819 citizen scientists who have taken part in the Exoplanet Explorers project! Because of you, we have been able to make several wonderful discoveries.