Abstract
Abstract Using the EVEREST photometry pipeline, we have identified 74 candidate ultra-short-period planets (USPs; orbital period P < 1 day) in the first half of the K2 data (Campaigns 0–8 and 10). Of these, 33 candidates have not previously been reported. A systematic search for additional transiting planets found 13 new multiplanet systems containing a USP, doubling the number known and representing a third (32%) of USPs in our sample from K2. We also identified 30 companions, which have periods from 1.4 to 31 days (median 5.5 days). A third (36 of 104) of the candidate USPs and companions have been statistically validated or confirmed in this work, 10 for the first time, including 7 USPs. Almost all candidates, and all validated planets, are small (radii R p ≤ 3 R ⊕) with a median radius of R p = 1.1 R ⊕; the validated and confirmed USP candidates have radii between 0.4 R ⊕ and 2.4 R ⊕ and periods from P = 0.18 to 0.96 days. The lack of candidate (a) ultra-hot-Jupiter (R p > 10 R ⊕) and (b) short-period-desert (3 ≤ R p ≤ 10 R ⊕) planets suggests that both populations are rare, although our survey may have missed some of the very deepest transits. These results also provide strong evidence that we have not reached a lower limit on the distribution of planetary radius values for planets at close proximity to a star and suggest that additional improvements in photometry techniques would yield yet more USPs. The large fraction of USPs in known multiplanet systems supports origins models that involve dynamical interactions with exterior planets coupled to tidal decay of the USP orbits.
Highlights
With about 200 planets known with orbital periods less than a day (Figure 1), it seems that planets can occupy every available nook and cranny of their stellar systems
We identified 28 systems for which imaging data are available using the ExoFOP website. (In a few well-studied cases, multiple contrast curves in similar wavelengths are available but not every curve was used, because additional curves were not needed to securely validate the planet.) Data came from six telescopes: WIYN, Keck, Gemini South, Gemini North, Palomar, and Southern Astrophysical Research (SOAR)
EPIC 201239401, from Campaign 1: the ultra-short-period planets (USPs) was reported by Vanderburg et al (2016b) and has signal-to-noise ratios (S/Ns) = 24, and we have newly identified a companion at P = 6.784 days
Summary
With about 200 planets known with orbital periods less than a day (Figure 1), it seems that planets can occupy every available nook and cranny of their stellar systems. These ultra-short-period planets (USPs) lie at the edge of orbital stability and have defied easy explanation, it seems unlikely that they formed where we find them (Jackson et al 2016). (3) Very few intermediate-sized planets (Rp = 3–10 R⊕) have been confirmed, a gap known as the short-period or sub-Jovian desert that extends out to P = 2–3 days (e.g., Lopez & Fortney 2014; Mazeh et al 2016).
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