Astronomers using NASA’s Transiting Exoplanet Survey Satellite and the High Resolution Echelle Spectrometer on the Keck I telescope at the W. M. Keck Observatory have discovered a system with three transiting exoplanets — including an ultra-short-period super Earth — around TOI-561, one of the oldest stars in our Milky Way Galaxy.
An artist’s rendition of the TOI-561 planetary system. Image credit: Adam Makarenko / W.M. Keck Observatory.
TOI-561 is a bright star located 280.5 light-years away in the constellation of Sextans.
The star is approximately 10 billion years old, and has a mass and size about 80% that of the Sun.
Otherwise known as TYC 243-1528-1, it belongs to a rare population of stars called the Galactic thick disk stars.
“Stars in this region are chemically distinct, with fewer heavy elements such as iron or magnesium that are associated with planet building,” said University of Hawaii postdoctoral researcher Lauren Weiss and colleagues.
TOI-561 hosts at least three small transiting planets, named TOI-561b, c, and d, and is one of the oldest, most metal-poor planetary systems discovered yet in the Milky Way.
The inner planet, TOI-561b, is a so-called super-Earth with an orbital period of only 0.44 days.
“For every day you’re on Earth, this planet orbits its star twice,” said co-author Dr. Stephen Kane, a planetary astrophysicist at the University of California, Riverside.
“Part of the reason for the short orbit is the planet’s proximity to its star, which also creates incredible heat.”
“Its estimated average surface temperature is over 1,727 degrees Celsius (2,000 degrees Kelvin) — much too toasty to host life as we know it today, though it may once have been possible.”
TOI-561b has a mass and a radius of 3.2 and 1.45 times that of the Earth, and a density of 5.5 g/cm3, consistent with a rocky composition.
“We calculated its density to be the same as our planet,” Dr. Kane said.
“This is surprising because you’d expect the density to be higher. This is consistent with the notion that the planet is extremely old.”
“TOI-561b is one of the oldest rocky planets yet discovered,” Weiss said.
“Its existence shows that the Universe has been forming rocky planets almost since its inception 14 billion years ago.”
The two outer planets, TOI-561c and d, have masses of 7 and 3 times that of the Earth, radii of 2.9 and 2.3 Earth radii, respectively.
They orbit their host star once every 10.8 and 16.3 days.
“Thanks to the bright host star, this multiplanet system is amenable to atmospheric follow-up with space-based telescopes,” the astronomers said.
“TOI-561b is expected to be a good eclipse target, while planets TOI-561c and d are promising targets for transmission spectroscopy.”
“Comparative atmospheric properties for the planets in this very metal-poor system would provide a unique test for planet formation scenarios.”
The discovery is reported in a paper in the Astronomical Journal.
_____
Lauren M. Weiss et al. 2021. The TESS-Keck Survey. II. An Ultra-short-period Rocky Planet and Its Siblings Transiting the Galactic Thick-disk Star TOI-561. AJ 161, 56; doi: 10.3847/1538-3881/abd409
