Planetary System Containing "Super-Earth" Discovered Around One Of The Galaxy's Oldest Stars

Artist’s rendition of TOI-561, one of the oldest, most metal-poor planetary systems discovered yet in the Milky Way galaxy. W. M. Keck Observatory/Adam Makarenko

The star TOI-561 is among the oldest in the Milky Way, and astronomers have now announced the discovery of three planets, making this one of the oldest known planetary systems. The age estimate for it places it at about 10 billion years old (double our own) with an uncertainty that could bring it down to 7 or up to 13, among the truly early years of the galaxy.

The three planets are super-Earth, larger and heavier than our planet, with the closest (TOI-561 b) having a radius 45 percent larger than Earth’s own. This exoplanet is rocky and weighs more than three times our own planet, but it has roughly the same density. This is very interesting.

Older rocky planets are less dense compared to the more recent objects of the same size, as there weren’t as many heavy elements available at the time they formed. This fact is consistent with the age estimate for this star. The system is very old.   

“TOI-561b is one of the oldest rocky planets yet discovered,” University of Hawaii postdoctoral fellow and team lead Dr Lauren Weiss said in a statement. “Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago.”

The second planet in the system is much larger and heavier, and it is believed to have a rocky core and a large gaseous atmosphere. The third planet is slightly smaller than the second one, but researchers couldn’t estimate its mass and derive other properties for it.

The planets are unlikely to have hosted life, given their proximity to their star. The closest orbits TOI-561 in less than half a day, and the furthest in 16 days. They add to the vast catalog of planetary objects we are beginning to discover around the stars in the Milky Way, and this discovery tells us that rocky planets might have been around for a very long time.

The discovery was possible thanks to NASA’s Transiting Exoplanet Survey Satellite, and with detailed observations from the W.M. Keck Observatory in Hawaii.

The system is located 280 light-years from Earth in the region of the Milky Way known as the thick disk, a location rich in a population of old stars. These stars tend to have fewer heavy elements such as iron and magnesium, which are important to build planets.

The discovery is accepted for publication in The Astronomical Journal, and was presented virtually at the 237th meeting of the American Astronomical Society.  

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