One of the oldest stars in the galaxy has a planet. Rocky planets were forming almost at the beginning of the universe

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Would it be surprising to find a rocky planet that dates back to the very beginning of the Universe? It should be. The first Universe lacked the heaviest elements needed to form rocky planets.

But astronomers have found one, right here in the Milky Way.

After the Big Bang, the Universe was only composed of light elements like 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 form in the hearts of stars. And the first stars didn’t form until about 200 million years after the Big Bang.

All extremely ancient planets, formed shortly after the start of the Universe, should be gaseous, not rocky. There was not enough time for the stars to seed the Universe with heavy elements for the rocky planets. Where was there?

The planet in question revolves around the star known as TOI 561. TOI stands for TESS Object of Interest, which means that it was spotted with NASA’s TESS (Transiting Exoplanet Survey Satellite). TOI 561 is one of the oldest stars in the Milky Way; about 10 billion years ago.

TESS discovered the planet and a team of researchers used tracking observations with the Keck Telescope to find out more. They presented their findings at the January 2021 meeting of the American Astronomical Society. They also published their findings in an article titled “The TESS-Keck Survey. II. A very short half-lived rocky planet and its siblings transiting the thick-disc galactic star TOI-561. It is published in the Astronomical Journal and the principal author is Dr. Lauren Weiss, postdoctoral fellow Beatrice Watson Parrent at the Institute of Astronomy, University of Hawaii.

TOI 561 is in rare company. It is one of the stars of what is called the thick galactic disk. The Galactic Thick Disc is made up almost entirely of ancient stars, which differ in chemistry and motion from the Thin Disc. Thick disc stars, including TOI 561, have a much lower metallicity than thin disc stars. It is therefore surprising to find a rocky planet in orbit.

Illustration showing the structural components of the Milky Way. The star TOI-561 is located in the thick disc (marked in red-orange), which contains a population of rare and older stars. While almost all known planets are found in the thin disc (marked in orange), the newly discovered rock and lava exoplanet orbiting TOI-561 is one of the earliest confirmed rock planets to orbit around a thick-disc galactic star. Credit: Kaley Brauer, MIT
Illustration showing the structural components of the Milky Way. The star TOI-561 is located in the thick disc (marked in red-orange), which contains a population of rare and older stars. While nearly all of the known planets are found in the thin disc (marked in orange), the newly discovered rock and lava exoplanet orbiting TOI-561 is one of the first confirmed rock planets to orbit around a thick-disc galactic star.
Credit: Kaley Brewers, MIT

“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, named TOI-561b, was discovered when it passed in front of its star. As the name suggests, TESS is designed to detect the trough in starlight as a planet passes a star in its field of view. Astronomers can measure the size of the planet by measuring the drop in light, and in this case, this indicates that the planet is about 1.5 Earth’s radius.

This is what the data on planetary transits looks like. It shows the trough measured in starlight as TOI 561b passes its star from TESS's point of view. Image credit: Weiss et al, 2021
This is what the data on planetary transits looks like. It shows the trough measured in starlight as TOI 561b passes its star from TESS’s point of view. Image credit: Weiss et al, 2021

The team used the Keck Observatory for follow-up observations. The Keck has a special instrument called the High Resolution Scale Spectrometer (HIRES) to confirm the detection of the planet. HIRES allows astronomers to measure the oscillation of 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 – that TOI 561b is a dense rocky planet rather than a gaseous planet. There are two other planets orbiting TOI-561, but they are both gas planets.

This study figure shows the apparent planet density versus planet radius for small planets with measured radii and masses, based on results from the NASA Exoplanet Archives. TOI 561 b, c and d are shown. The planets in our solar system are shown for comparison. Image credit: Weiss et al, 2021.
This study figure shows the apparent planet density versus planet radius for small planets with measured radii and masses, based on results from the NASA Exoplanet Archives. TOI 561 b, c and d are shown. The planets in our solar system are shown for comparison. Image credit: Weiss et al, 2021.

The origin of the old stars in the thick galactic disk is unclear. They could be the remains of an ancient galaxy engulfed by the Milky Way. Or they could be the first stars to form in the Milky Way. Or it could be something else. No one is sure.

As a planet orbiting a 10 billion year old ancient star, it has been through a lot. The wandering movement of the stars in the disk sometimes takes them above the galactic plane. An observer on TOI 561b would have had a stunning view of the magnificent spiral structure of the Milky Way. “I wonder what view of the night sky would have been accessible from the rocky planet during its history,” Weiss said.

Artistic interpretation of TOI-561, one of the oldest and most metal-poor planetary systems discovered to date in the Milky Way galaxy. This 10 billion year old system has a hot, rocky exoplanet (center) that is one and a half times the size of Earth as well as two gas planets (to the left of the rocky planet) that are about twice the size. than Earth. . Credit: WM Keck Observatory / Adam Makarenko
Artist’s representation of TOI-561, one of the oldest and most metal-poor planetary systems discovered to date in the Milky Way galaxy. This 10 billion year old system has a hot, rocky exoplanet (center) that is one and a half times the size of Earth as well as two gas planets (to the left of the rocky planet) that are about twice the size. than Earth. . Credit: WM Keck Observatory / Adam Makarenko

But there were probably no observers. Not now, anyway. Although the long history of the planet is unknown, it is too hot for life these days. TOI 561b in an ultra short period planet (USP). It orbits TOI 561 twice a day on Earth at a distance that keeps the surface around 2000 degrees Kelvin (1721 C; 3140 F.) So even though it is a rocky planet, this rock is probably magma on the surface.

What does this discovery mean? Well, it would be surprising if it was the only one. This 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 detected around a galactic thick disc star. The Kepler mission found five sub-terrestrial size planets orbiting the Kepler 444 triple star system. Kepler 444 is estimated to be just 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 have a blazing heat.

There is also LHS 1815b. It orbits a dwarf star M in the thick disk and was discovered in early 2020. It is a rocky super-Earth and has a blazing surface temperature due to its proximity to its star.

An illustration of LHS 1815b, an ancient rocky exoplanet orbiting a star in the thick galactic disk. Image Credit: NASA
An illustration of LHS 1815b, an ancient rocky exoplanet orbiting a star in the thick galactic disk. Image Credit: NASA

There is still a mystery in this discovery. Not so much on the planet itself, but on the thick disk. There is disagreement and uncertainty over the nature of the galactic thick disk itself. Some astronomers believe it doesn’t even exist as a separate structure. In 2012, a group of astronomers published an article in the Astrophysical Journal titled “The Milky Way Has No Distinct Thick Disc.” They argued that there is no distinct thick disc and say their data showed that “… the Milky Way has a continuous and monotonous distribution of disc thicknesses: there is no” thick disc. ‘Reasonably characterized as a separate component.’

No one denies the existence of the ancient stars themselves or the ancient rocky planets orbiting them. But what the existence of these planets means to our understanding of the Universe is not yet clear.

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