TOI-561

TOI 561 is an old, metal-poor Sun-like star, known to have multiple small planets.^[3][2][4][5] It is an orange dwarf, estimated to be 10.5 billion years old, and about 79% the mass and 85% the radius of Sol, Earth's sun.^[6][5]

In January 2021 a team led by Lauren Weiss of the University of Hawaii at Manoa announced that, using data from NASA's Transiting Exoplanet Survey Satellite, they had found a Super-Earth in a very close orbit, as well as two outer Sub-Neptunes.^[7][8][6][3] The innermost planet, TOI-561b, orbits in under one Earth day.^[2][3] Another team led by Gaia Lacedelli of the University of Padua independently announced the discovery in a paper published in December 2020.^[2] However, the two papers disagree on the structure of the system. While the innermost two planets were confirmed from TESS data by both papers, Weiss proposes only a single third planet in a 16.3-day orbit, while Lacedelli argues that the system instead contains two further planets, in wider orbits of 25.6 and 77 days.^[2]

Discovery and Nomenclature[]

TOI-561 is also designated 2MASS J09524454+0612589 in the 2MASS catalog and TIC 377064495 in the TESS Input Catalog. When its planets were first identified, it was renamed TOI-561, with TOI standing for "TESS Object of Interest".^[9]

The planetary system was independently confirmed and characterized by Lacedelli et al 2020 and Weiss et al 2021. Lacedelli et al found evidence for four exoplanets: the Ultra-Short-Period (USP) Super-Earth TOI-561b, and three Sub-Neptunes designated TOI-561c, d, and e. The two planets TOI-561d and e were originally listed as a single planet with a period of 16 days on ExoFOP, but Lacedelli could not detect a planet in that orbit using radial velocity data from HARPS, and instead interpreted it as two separate transits coming from planets correlating with periods of 25.6 and 77.2 days found by HARPS.^[2]

In January 2021, Lauren Weiss and her team's study on TOI-561 was published. Unlike Lacedelli, they kept the 16-day signal and designated it TOI-561d;^[3] it is referred to as TOI-561f on NASA's Exoplanet Archive to avoid confusion with the TOI-561d from Lacedelli's paper.^[9]

Characteristics[]

TOI-561 is a yellow or orange star approximately 80% the size of the Sun. According to Lacedelli, it is 85% the radius and 79% the mass of the Sun, with a temperature of 5455 K.^[2] Weiss found the star to be 83.2% the radius and 80.5% the mass of the Sun, with a temperature of 5326 K and a luminosity just over half that of the Sun.^[3] Both teams found that TOI-561 has an extremely low abundance of metals, or any element heavier than hydrogen or helium, and is very old; Weiss calculates an age of roughly 10 billion years. It is also a part of the Galactic Thick-Disk and is the first of those stars to have confirmed transiting exoplanets.^[3]

Planetary System[]

Depending on the study, TOI-561 has either 3 (Weiss) or 4 (Lacedelli) planets. The discrepancy comes from different interpretations of the two transit events associated with TOI-561d in Weiss 2020. Only two transits were observed by TESS, and a third transit for a 16-day period would have occurred in the middle of a data gap.^[3] Weiss attributes the two transits to that of a single Sub-Neptune sized planet. However, in the radial velocity analysis by Lacedelli 2020, the 16-day signal is not recovered, but there are two additional signals of 26 and 77 days that they attribute to one of the two transits each.^[2] The follow-up study in 2022 has confirmed the architecture of 4-planet system. Additional, 5th planet on the 473⁺³⁶
₋₂₅ days orbit is suspected.^[10]