Ultra-short-period (USP) planets refer to a class of exoplanets (usually radius smaller than 2 R) with periods less than 1.0 day. Since the earliest examples were discovered back in the late 2000’s, more than 100 such USP planets have been reported to date. Now, a team of international astronomers presented observations of two bright M dwarfs (TOI-1634 and TOI-1685: J = 9.5 − 9.6) hosting ultra-short period (USP) planet candidates, identified by the TESS mission. The two stars are similar in temperature, mass, and radius, and the planet candidates are both super-Earth-sized (1.25 R < Rp < 2.0 R). For both systems, light curves from the ground-based photometry exhibit planetary transits, whose depths are consistent with those by the TESS photometry.
They also refined the transit ephemerides based on the ground-based photometry, finding the orbital periods of P = 0.9893457 day and P = 0.6691425 day for TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV) observations using the IRD spectrograph on the Subaru 8.2m telescope, they confirmed the planetary nature of the TOIs, and measure their masses: 9.94 M and 3.84 M for TOI-1634b and TOI-1685b, respectively, when the observed RVs are fitted with a single-planet circular-orbit model.
Combining those with the planet radii of Rp = 1.773 R (TOI-1634b) and 1.463 R (TOI-1685b), they found that both USP planets have mean densities consistent with an Earth-like internal composition, which is typical for small USP planets.
They concluded that, TOI-1634b is currently the most massive USP planet in this category, and it resides near the radius valley, which makes it a benchmark planet in the context of testing the formation scenarios for USP planets. While, excess scatter in the RV residuals for TOI-1685 suggests the presence of a possible secondary planet or unknown activity/instrumental noise in the RV data, but further observations are required to check those possibilities.
Featured image: TESS photometric apertures and 3′ × 3′ archival images for TOI-1634 (top) and TOI-1685 (bottom). The archival images are scanned photographic plates using the RG610 filter and the IIIaF emulsion, which were originally obtained as part of the POSSII-F survey on September 18, 1988 (TOI-1634) and October 6, 1989 (TOI-1685). The Gaia DR2 positions (epoch J2015.5) of the target stars are indicated by magenta circles, and other sources by gray circles © Hirano et al.
Reference: Teruyuki Hirano, John H. Livingston, Akihiko Fukui, Norio Narita, Hiroki Harakawa, Hiroyuki Tako Ishikawa, Kohei Miyakawa, Tadahiro Kimura, Akifumi Nakayama, Naho Fujita, Yasunori Hori, Keivan G. Stassun, Allyson Bieryla, Charles Cadieux, David R. Ciardi, Karen A. Collins, Masahiro Ikoma, Andrew Vanderburg, Thomas Barclay, C. E. Brasseur, Jerome P. de Leon, John P. Doty, René Doyon, Emma Esparza-Borges, Gilbert A. Esquerdo, Elise Furlan, Eric Gaidos, Erica J. Gonzales, Klaus Hodapp, Nobuhiko Kusakabe, Masayuki Kuzuhara, David Lafrenière, David W. Latham, Bob Massey, Mayuko Mori, Felipe Murgas, Jun Nishikawa, Taku Nishiumi, Masashi Omiya, Martin Paegert, Enric Palle, Hannu Parviainen, Samuel N. Quinn, Steve B. Howell, Keisuke Isogai, Shane Jacobson, Jon M. Jenkins, Eric L. N. Jensen, Kiyoe Kawauchi, Takayuki Kotani, Tomoyuki Kudo, Seiya Kurita, Takashi Kurokawa, George R. Ricker, Richard P. Schwarz, Sara Seager, Peter Tenenbaum, Yuka Terada, Roland K. Vanderspek, Noriharu Watanabe, Joshua N. Winn, “Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions”, ArXiv, pp. 1-27, 2021. https://arxiv.org/abs/2103.12760
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