Astronomers have found an Earth-size (but not at all Earth-like) planet around an ancient star that has a nice high view of our galaxy.

This artist’s rendition shows TOI-561, one of the oldest, most metal-poor stars known in the Milky Way Galaxy. This star hosts a hot, rocky exoplanet (center) as well as two gas planets (to the left).
W. M. Keck Observatory / Adam Makarenko

Astronomers have discovered three planets orbiting a star about 10 billion years old — one of them rocky. The star, TOI 561 (meaning it was the 561st object of interest from the Transiting Exoplanet Survey Satellite), is in our galaxy’s older thick disk, which means its planets have a nice view from on high of the Milky Way’s spiral.

The star has three planets, with diameters 1.45, 2.9, and 2.3 times Earth’s. The innermost one is rocky, with three times Earth’s mass, but on a period of 0.44 days, it’s anything but Earth-like. Its dayside surface temperature is around 2500K (4,000°F). That’s at least twice as hot as Earth’s magma, and it’s surely molten. What it actually looks like is uncertain, because as lead scientist Lauren Weiss (University of Hawaii, Manoa) notes, “It exceed temperatures where geophysicists have made lava in the lab.”

Thick Disk of the Milky Way
Illustration showing the structural components of the Milky Way galaxy. The star TOI 561 is located in the thick disk (marked in red-orange), which contains a sparse, older population of stars.
Kaley Brauer / MIT

Astronomers have found planets around old stars before, and even around chemically poor stars that have a paucity of heavy elements. Yet the mere fact that this planet came to be is of interest to astronomers. “We now have evidence that the universe has been forming rocky planets for the last 10 billion years, more than double the age of our own solar system, and nearly since the origin of the universe itself,” Weiss says.


Image of Rod


January 14, 2021 at 2:12 pm

TOI-561 b is very interesting. I read some other reports on this exoplanet too, the arxiv paper is here, The TESS-Keck Survey II: An Ultra-Short Period Rocky Planet and its Siblings Transiting the Galactic Thick-Disk Star TOI-561,, "We report the discovery of TOI-561, a multi-planet system in the galactic thick disk that contains a rocky, ultra-short period planet (USP)."

My observation. Finding rocky exoplanets orbiting very old stars with low metal content is surprising to the accretion disk model. Our galaxy's gas at 10 or 11 billion years ago is supposed to be largely metal free so the find here is *surprising*, see

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