TOI-561
Old, metal-poor star with multiple small exoplanets.
TOI-561 is an old, metal-poor, Sun-like orange dwarf in Sextans, near the border with Leo. It hosts multiple small exoplanets, including an ultra-short-period Super-Earth. The system is notable for being the first with confirmed transiting exoplanets belonging to the Galactic thick disk.
Quick Facts
- Constellation
- Sextans
- Age
- 10.5 billion years
- Temperature
- 5455 K (Lacedelli); 5326 K (Weiss)
- Metallicity
- extremely low
- Distance
- near border with Leo
Facts from the source article.
Discovery and nomenclature
TOI-561 is also known as 2MASS J09524454+0612589 and TIC 377064495. It was renamed TOI-561 after its planets were identified. The planetary system was independently confirmed by Lacedelli et al. and Weiss et al. Lacedelli found four exoplanets: the ultra-short-period Super-Earth TOI-561 b and three Sub-Neptunes (c, d, e). Two planets originally listed as a single 16-day planet on ExoFOP were reinterpreted by Lacedelli as separate planets with periods of 25.6 and 77.2 days. Weiss kept the 16-day signal as TOI-561 d, which is called TOI-561 f on NASA's Exoplanet Archive to avoid confusion.
Characteristics
TOI-561 is a yellow or orange star about 80% the size of the Sun. Its radius is 85% and mass 79% of the Sun per Lacedelli, with a temperature of 5455 K. Weiss measured 83.2% radius, 80.5% mass, 5326 K, and luminosity just over half the Sun's. The star has extremely low metal abundance and is very old, around 10 billion years. It belongs to the Galactic thick disk.
Planetary system
The system has either three (Weiss) or four (Lacedelli) planets, depending on interpretation of two transit events. Weiss attributes them to a single Sub-Neptune with a 16-day period; Lacedelli assigns them to planets with 25.6- and 77-day orbits. A 2022 study confirmed the four-planet architecture, with a possible fifth planet on a 473-day orbit. Observations from CHEOPS and TESS in 2024 refined the parameters, confirming four transiting planets and a fifth non-transiting candidate. TOI-561 b is an ultra-short-period Super-Earth with radius about 1.4 Earths and an orbital period under 11 hours, giving an equilibrium temperature of 2480 ± 200 K. Weiss found a mass of 3.2 Earths and density 5.5 g/cm³, implying a rocky but iron-poor composition. Lacedelli found a mass of 1.59 Earths and density 3.0 g/cm³, suggesting a water-world composition of 50% or more water. If water-rich, TOI-561 b may have a puffy steam atmosphere and would support formation scenarios of Super-Earths migrating inward from beyond the snow line.
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