Ultra-short-period Planets in K2. III. Neighbors are Common with 13 New Multiplanet Systems and 10 Newly Validated Planets in Campaigns 0-8 and 10

Utilizing the EVEREST photometry pipeline, 74 ultra-short-period planets (USPs) candidates with orbital periods less than a day were identified in the initial data from K2's campaigns, including 33 new candidates. A further search revealed 13 new multi-planet systems containing a USP, doubling the known number and accounting for a third of the K2's USP sample. Additionally, 30 companions with periods ranging from 1.4 to 31 days were identified. Out of these, 36 candidates including 10 new ones, have been statistically validated or confirmed, with seven being USPs. Almost all candidates, especially validated ones, are small with a median radius of 1.1 Earth radii (R⊕), showcasing a range between 0.4 R⊕ and 2.4 R⊕ with periods ranging from 0.18 to 0.96 days. The absence of ultra-hot-Jupiter and short-period desert planets suggests such populations are rare, though some deep transits might have been missed. The findings also indicate that there isn't a lower limit on planetary radius values for close-orbit planets, hinting at the possibility of discovering more USPs with improved photometry techniques. A significant portion of USPs in known multi-planet systems supports theories of origin involving dynamical interactions with exterior planets and tidal decay of USP orbits. This work was published by the American Astronomical Society in 2021.