Would it be surprising to find a rocky planet that was from the very early universe? It should be. The early universe lacked the heavier elements necessary to form rocky planets.
But astronomers found one right here in the Milky Way.
After the Big Bang, the universe consisted only of light elements such as hydrogen and helium with a little lithium. Rocky planets require heavier elements like carbon, oxygen, and iron, which astronomers call metals. These heavier elements can only be formed in the hearts of the stars. And the first stars did not form until about 200 million years after the Big Bang.
All extremely old planets that formed not long after the universe began should be gaseous, not rocky. The stars did not have enough time to seed the universe with heavy elements for rocky planets. Or was there?
The planet in question orbits the star known as TOI 561. TOI stands for TESS Object of Interest, which means that it was discovered by NASA’s TESS (Transiting Exoplanet Survey Satellite). TOI 561 is one of the oldest stars in the Milky Way; about 10 billion years old.
TESS found the planet and a team of researchers used follow-up observations with the Keck telescope to learn more about it. They presented their results at the American Astronomical Society meeting in January 2021. They also published their results in an article entitled “The TESS-Keck Survey. II. An ultra-short Rocky Planet and its siblings traversing the galactic thick-disk star TOI-561. “It was published in the Astronomical Journal and the lead author is Dr. Lauren Weiss, Beatrice Watson Parrent Postdoctoral Fellow at the Department of Astronomy, University of Hawaii.
TOI 561 is in rare company. It is one of the stars in the so-called galactic thick disk. The galactic thick disk consists almost entirely of ancient stars, the chemistry and movement of which differ from the thin disk. Thick disk stars, including TOI 561, have much less metallicity than stars in the thin disk. So it’s surprising to find a rocky planet orbiting it.
Figure shows the structural components of the Milky Way. The star TOI-561 is in the thick disk (marked red-orange) that contains a rare, older population of stars. While almost all known planets are in the thin disk (marked orange), the newly discovered stone-lava exoplanet TOI-561 is one of the first confirmed rocky planets orbiting a galactic thick disk star.
Credit: Kaley Brewer, MIT
“The rocky planet orbiting TOI-561 is one of the oldest rocky planets discovered to date. Its existence shows that the universe has formed rocky planets almost since its inception 14 billion years ago, ”lead author Weiss said in a press release.
The planet, named TOI-561b, was discovered when it was crossing in front of its star. As the name suggests, TESS was developed to detect the ingress of starlight when a planet passes in front of a star in its field of view. Astronomers can measure the size of the planet by measuring the fall in light. In this case, it indicates that the planet is about 1.5 earth radii.
This is what planet transit data looks like. It shows the inclination of the starlight measured when TOI 561b passes in front of its star from the perspective of TESS. Photo credit: Weiss et al., 2021
The team used the Keck observatory for follow-up observations. The Keck has a special instrument, the High Resolution Echelle Spectrometer (HIRES), to confirm the recognition of the planet. With HIRES, astronomers can measure the wobbling in the star caused by the planet’s gravitational tug. This measurement shows the mass of the planet. In this case, the mass is large enough – three times that of Earth – that TOI 561b must be a dense rocky rather than a gaseous planet. There are two other planets orbiting TOI-561, but both are gas planets.
This illustration from the study shows the planet’s bulk density versus planetary radius for small planets with measured radii and masses, based on results from the NASA Exoplanet Archive. TOI 561 b, c and d are shown. For comparison, planets in our solar system are shown. Photo credit: Weiss et al., 2021.
The origin of the ancient stars in the galactic thick disk is unclear. They could be the remains of an ancient galaxy that was swallowed up by the Milky Way. Or they could be the first stars to form in the Milky Way. Or it could be something else. Nobody is safe.
As a planet orbiting an ancient 10 billion year old star, it has been through a lot. The wandering movement of the stars in the disk sometimes leads them across the galactic plane. An observer on TOI 561b would have had a breathtaking view of the beautiful spiral structure of the Milky Way. “I wonder what view of the night sky from the rocky planet would have been available during its history,” Weiss said.
Artistic rendering of TOI-561, one of the oldest and most metal-poor planetary systems discovered to date in the Milky Way. This 10 billion year old system has a hot, rocky exoplanet (center) that is one and a half times the size of Earth and two gas planets (to the left of the rocky planet) that are roughly twice the size of Earth. Photo credit: WM Keck Observatory / Adam Makarenko
But there were probably no observers. At least not now. Although the planet’s long history is unknown, it is too hot to live in the present. TOI 561b in an ultra-short planet (USP). It orbits TOI 561 twice a day on Earth at a distance that keeps the surface at about 2000 degrees Kelvin (1721 ° C). So while this is a rocky planet, this rock is likely magma on the surface.
What does this discovery mean? Well it would be surprising if it were the only one. It is likely an indication that there is a population of ancient, rocky planets orbiting ancient stars.
In fact, TOI 561b is not the first planet to be discovered around a galactic thick disk star. The Kepler mission found five subterranean planets orbiting the triple star system Kepler 444. Kepler 444 is estimated to be a little over 11 billion years old, and its five planets are all rocky planets smaller than Venus. They are also very close to their star and hot.
There is also LHS 1815b. It orbits an M dwarf star in the thick disk and was discovered in early 2020. It is a rocky super-earth and has a bubbly surface temperature due to its proximity to its star.
An illustration of LHS 1815b, an ancient rocky exoplanet orbiting a star in the galactic thick disk. Photo credit: NASA
There is still a mystery to this discovery. Not so much over the planet itself, but over the thick disk. There is disagreement and uncertainty about the nature of the galactic thick disk itself. Some astronomers believe that it does not even exist as a structure on its own. In 2012, a group of astronomers published an article in the Astrophysical Journal entitled “The Milky Way Does Not Have a Distinct Thick Disc”. They argued that there is no distinct thick disk, and said their data showed that “… the Milky Way has a continuous and monotonous distribution of disk thicknesses: there is no” thick disk “that is reasonably characterized as a separate component.”
Nobody denies the existence of the ancient stars themselves or the ancient rocky planets that orbit them. But what the existence of these planets means for our understanding of the universe is not yet clear.
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