[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.
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.
[2][3] Another team led by Gaia Lacedelli of the University of Padua independently announced the discovery in a paper published in December 2020.
[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.
[10] The orbital parameters were refined with additional observations from CHEOPS and TESS in 2024, further confirming four transiting planets and a fifth non-transiting candidate.
It has an extremely short orbital period of under 11 hours, less than half of an Earth day, resulting in an equilibrium temperature of 2,480 ± 200 K (2,207 ± 200 °C; 4,004 ± 360 °F).
[3] Lacedelli 2020, on the other hand, found a mass of only 1.59 Earths and a density of 3.0 grams per cubic centimetre, abnormally low for a planet of its size and suggesting a composition made of 50% or more of water.
If it is an extremely water-rich world, TOI-561 b would prove formation scenarios about Super-Earths forming beyond the "Snow Line" and migrating inwards.
[3] With a radius of 2.9 Earths and a mass of 5.4 to 7.0 Earths, the planet has a Neptune-like density of 1.3 to 1.6 grams per cubic centimetre, implying that it is a small gas planet with a similar composition, albeit far hotter and closer to its star than our system's ice giants.