Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. II. Planetary Systems Observed During Campaigns 1–7

Abstract

We recently used near-infrared spectroscopy to improve the characterization of 76low-mass stars around which K2 had detected 79candidate transiting planets. 29 of these worlds were new discoveries that had not previously been published. We calculate the false positive probabilities that the transit-like signals are actually caused by nonplanetary astrophysical phenomena and reject five newtransit-like events and three previously reported events as false positives. We also statistically validate 17planets (7 of which were previously unpublished), confirm the earlier validation of 22planets, and announce 17newly discovered planet candidates. Revising the properties of the associated planet candidates based on the updated host star characteristics and refitting the transit photometry, we find that our sample contains 21planets or planet candidates with radii smaller than 1.25 R ⊕ , 18super-Earths (1.25-2 R ⊕ ), 21small Neptunes (2-4 R ⊕ ), threelarge Neptunes (4-6 R ⊕ ), and eightgiant planets (>6 R ⊕ ). Most of these planets are highly irradiated, but EPIC206209135.04 (K2-72e, ) orbit within optimistic habitable zone boundaries set by the "recent Venus" inner limit and the "early Mars" outer limit. In total, our planet sample includes eight moderately irradiated 1.5-3 R ⊕ planet candidates (F p 20 F ⊕ ) orbiting brighter stars (Ks<11) that are well-suited for atmospheric investigations with the Hubble, Spitzer, and/or James Webb Space Telescopes. Five validated planets orbit relatively bright stars (Kp<12.5) and are expected to yield radial velocity semi-amplitudes of at least 2 m s −1 . Accordingly, they are possible targets for radial velocity mass measurement with current facilities or the upcoming generation of red optical and near-infrared high-precision RV spectrographs.

275 Candidates and 149 Validated Planets Orbiting Bright Stars in K2 Campaigns 0–10

“…We also found that inputting 2MASS photometry into isochrones along with the aforementioned stellar parameters had no noticeable effect on the resulting stellar radius and mass values (or their uncertainties). Although isochrones have been found to underestimate stellar radii before (Dressing et al 2017a(Dressing et al , 2017bMartinez et al 2017), this effect is confined to M dwarfs and late-K dwarfs, which are largely excluded from our host star sample since we do not consider stars with effective temperatures below 4250 K (see Section 4.2). The derived stellar masses and radii are reported in Table 2.…”

“…K2 has led to the discovery of numerous candidate and confirmed planets Foreman-Mackey et al 2015;Montet et al 2015;Vanderburg et al 2015bVanderburg et al , 2016bAdams et al 2016;Barros et al 2016;Crossfield et al 2016;Schlieder et al 2016;Sinukoff et al 2016;Pope et al 2016;Dressing et al 2017b;Hirano et al 2017;Martinez et al 2017), as well as to the identification of planets orbiting rare types of stars, including particularly bright nearby dwarf stars Vanderburg et al 2016a;Christiansen et al 2017;Crossfield et al 2017;Niraula et al 2017;Rodriguez et al 2017aRodriguez et al , 2017b, young, pre-main-sequence stars (Mann et al 2016;David et al 2016), and disintegrating planetary material transiting a white dwarf (Vanderburg et al 2015a).…”

Precise Masses in the WASP-47 System

“…The Kepler era of discovery was instead expected to give way to a new era in which the K2 mission would largely find more of the same kinds of planets that had been previously discovered, just orbiting brighter stars or in different environments. While K2 has certainly delivered in that regard, finding small planets around nearby stars well-suited to detailed characterization (Vanderburg et al , 2016aSinukoff et al 2016;Crossfield et al 2017; Rodriguez et al 2017), temperate planets around M-dwarfs Petigura et al 2015;Dressing et al 2017aDressing et al , 2017bMartinez et al 2017), and planets in open clusters (David et al 2016;Libralato et al 2016;Mann et al 2016aMann et al , 2016bObermeier et al 2016;Mann et al 2017), K2 is still delivering new and unexpected discoveries, like the unique architecture of the WASP-47 system, that are yielding fundamental insights about the formation and evolution of planetary systems.…”

Planetary Candidates from K2 Campaign 16

“…Dressing et al (2017) validated two of our high-quality C16 candidates, 212069861.01 (K2-123b) and 212154564.01 (K2-124b); another candidate 212110888.01 is a previously known hot Jupiter K2-34b (Hirano et al 2016;Lillo-Box et al 2016); and our lower-priority candidate 211969807.01 was validated as K2-104b (Mann et al 2017). One more low-quality candidate, 211946007.01, was confirmed to be a transiting brown dwarf (Gillen et al 2017).…”