Researchers using data collected with NASA’s TESS spacecraft and the Keck Observatory in Hawaii have found a rocky planet. That story normally wouldn’t be news, but this planet orbits an extremely old star in our Milky Way that is in what we call the “thick disk”. This means that the star is located within the red-orange oval in the graphic rather than the thin yellow oval where the majority of stars and planets exist.

The stars in the thick disk are unusual. They are far older, on the order of 10 billion years old. They are considered metal-poor, and in this instance, TOI-561 has about 50% the metallicity of our Sun, which used to mean we didn’t expect them to have rocky planets in their orbit. And finally, they have an amazing view of the Milky Way since they out of the majority of the stars and dust, although it’s unlikely anyone is living in the TOI-561 system at this point.

Currently, measurements indicated that this little planet orbits TOI-561 twice for every Earth day and is so close that the surface temperature is over 1700 degrees Celsius (about 3100 degrees Fahrenheit). The planet was found using the transit method, so we had to luck out into the science here a bit, and even with the proper alignment, this planet only caused a 0.025% drop in the star’s brightness as it passed by.

The metal-poor content of the star seems to have led to a lower amount of iron in TOI-561’s rocky world than Earth has, despite the fact that it’s three times the mass. Additionally, it’s not the sole planet in the system. There are two other planets in orbit around TOI-561, both about twice the Earth’s radius but unlikely to be rocky due to their size and mass.

The paper has been accepted for publication in the Astronomical Journal, and lead author Dr. Lauren Weiss sums it up, “The rocky planet orbiting TOI-561 is one of the oldest rocky planets yet discovered. Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago.”

I’m not sure we’ll ever reach a day where we completely understand and can predict how and where planets form. I’m honestly not sure I want to reach that day since the science here continues to be interesting and always shifting a little.

More Information

Keck Observatory press release 

University of Hawai’i press release 

UC Riverside press release 

“The TESS-Keck Survey. II. An Ultra-short-period Rocky Planet and Its Siblings Transiting the Galactic Thick-disk Star TOI-561,” Lauren M. Weiss et al., 2021 January 11, The Astronomical Journal