Would it be surprising to find a rocky planet that dates back to a very early universe? Must be. The early universe lacked the heavier elements needed to form rocky planets.
But astronomers have found one right here in the Milky Way.
After the Big Bang, the universe is made up of nothing but light elements like hydrogen and helium, with little lithium. Rocky planets require heavier elements such as carbon, oxygen and iron, which astronomers call metals. These heavier elements can only form in the hearts of the stars. And the first stars formed only about 200 million years after the Big Bang.
All extremely ancient planets formed soon after the beginning of the universe must be gaseous, not rocky. There was not enough time for the stars to flood the universe with heavy elements for rocky planets. Or was he there?
The planet in question orbits the star known as TOI 561
TESS discovered the planet and a team of researchers used follow-up observations with the Keck telescope to learn more about it. They presented their findings at a meeting of the American Astronomical Society in January 2021. They also published their findings in a report entitled “The TESS-Keck Study. II. A rocky planet with an ultra-short period and its siblings passing through the galactic star with a thick disk TOI-561. “Published in the Astronomical Journal, and the lead author is Dr. Lauren Weiss, Beatrice Watson Parent, a PhD student at the Institute of Astronomy at the University of Hawaii.
TOI 561 is a rare company. This is one of the stars in the so-called galactic thick disk. The galactic thick disk is composed almost entirely of ancient stars whose chemistry and motion are different from that of the thin disk. Thick disk stars, including the TOI 561, have much lower metallicity than stars in the thin disk. So finding a rocky planet orbiting it is surprising.
“The rocky planet orbiting TOI-561 is one of the oldest rocky planets ever discovered. Its existence shows that the universe has formed rocky planets almost since its creation 14 billion years ago, “lead author Weiss said in a press release.
The planet, called TOI-561b, was discovered when it transited in front of its star. As its name suggests, TESS is designed to detect sinking in starlight as a planet passes in front of a star in its field of vision. Astronomers can estimate the size of the planet by measuring the drop in light, and in this case this indicates that the planet has about 1.5 Earth radii.
The team used the Keck Observatory for follow-up observations. Keck has a special instrument called a high-resolution Echelle spectrometer (HIRES) to confirm the discovery of the planet. HIRES allows astronomers to measure the oscillations in the star caused by the planet’s gravitational tug. This measurement reveals the mass of the planet. In this case, the mass is large enough – three times that of Earth, so TOI 561b should be a dense rocky planet, not gaseous. There are two other planets orbiting TOI-561, but both are gas planets.
The origin of the old stars in the galactic thick disk is unclear. They may be the remains of an ancient galaxy swallowed by the Milky Way. Or they may be the first stars to form in the Milky Way. Or it could be something else. No one is sure.
As a planet orbiting an ancient 10 billion star, it has experienced a lot. The wandering motion of the stars in the disk sometimes takes them above the galactic plane. An observer on the TOI 561b would be given a stunning view of the beautiful spiral structure of the Milky Way. “I wonder what view of the night sky would be accessible from the rocky planet in its history,” Weiss said.
But there were probably no observers. Not now, anyway. Although the long history of the planet is unknown, it is now too hot for life. TOI 561b in an ultra-short period planet (USP). It orbits TOI 561 twice a day on Earth at a distance that maintains a surface of about 2,000 degrees Kelvin (1721 C; 3140 F.) So while it is a rocky planet, this rock is probably magma on the surface.
What does this discovery mean? Well, it would be surprising if he was the only one. It is probably an indication that there is a whole population of ancient rocky planets orbiting ancient stars.
In fact, TOI 561b is not the first planet discovered around a galactic star with a thick disk. Kepler’s mission has discovered five underground planets orbiting the triple star system Kepler 444. Kepler 444 is expected to be just over 11 billion years old, and its five planets are rocky planets smaller than Venus. They are also very close to their star and are hot.
There is also the LHS 1815b. It orbits the thick disk M-dwarf and was discovered in early 2020. It is a rocky super-Earth and has a bubble surface temperature due to its proximity to its star.
There is still some mystery in this discovery. Not so much over the planet itself as 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 separate structure. In 2012, a group of astronomers published an article in the Astrophysical Journal entitled “The Milky Way has no different thick disk.” They claim that there is no distinct thick disk and say that their data show that “… the Milky Way has a continuous and monotonous distribution of disk thicknesses: there is no ‘thick disk’ that can reasonably be characterized as a separate component.”
No one denies the existence of the ancient stars themselves or the ancient rocky planets that orbit them. But what the existence of these planets means to our understanding of the universe is not yet clear.