It turns out that planets can actually live a very long time.
Around one of the galaxy’s oldest stars, an orange dwarf named TOI-561 just 280 light-years away, astronomers have found three orbiting exoplanets – one of which is a rocky world 1.5 times the size of Earth and whips around the star. on a violent neck 10.5-hour course.
Clearly, an exoplanet so close to its star is unlikely to be habitable, even though it is rocky like Earth, Venus, and Mars. It would have a temperature of 2,480 Kelvin, temporarily locked with a magma sea on the permanent day side.
But the TOI-561 system, planets and all, is one of the oldest ever seen at an estimated age of about 10 billion years.
It is more than twice as old as the solar system, almost as old as the universe itself, and evidence that rocky exoplanets can remain stable for a very long time.
“Its existence shows that the universe has formed rocky planets almost since its inception 14 billion years ago.”
The three planets, named TOI-561 b, TOI-561 c and TOI-561 d, were identified by NASA’s planetary orbiting space telescope, TESS. TESS stares at sections of the sky, looking for periodic, faint drops in the light from distant stars. These are passages when a planet passes between us and its star.
From these data and follow-up observations, astronomers were able to determine the periods and sizes of the orbit of the three exoplanets.
TOI-561 d, the outermost, is about 2.3 times the size of Earth with a rotation time of 16.3 days. TOI-561 c is 2.9 times the size of Earth with a rotation time of 10.8 days. And the TOI-561 b is 1.45 times the size of the Earth with a rotation time of just over 10.5 hours.
The team also conducted radial velocity measurements. When planets orbit a star, that star does not sit still. Each exoplanet exerts its own gravity on the star, resulting in a small complex dance that compresses and stretches the star’s light as it moves toward and away from us as we observe it.
If we know the mass of the star, we can observe how much the star moves in response to the gravity of an exoplanet and calculate the mass of the exoplanet. Based on this, the researchers calculated that the TOI-561b is approximately three times the mass of the Earth.
But its density is about the same as that of the earth, about five grams per. Cubic centimeters.
“This is surprising because you would expect the density to be higher,” said planetary astrophysicist Stephen Kane of the University of California, Riverside. “This is in line with the notion that the planet is extremely ancient.”
This is because the heavier elements in the universe – metals heavier than iron – are thrown into the hearts of the stars, into the supernovae at the end of the life and collision of a massive star. Only when stars are dead and scattered these elements into space can they be taken up in other objects.
So the very oldest stars in the universe are very poor in metals. TOI-561, for example, has low metallicity. And all the planets that formed in the former universe should also have low metallicity.
Previous studies have suggested that there is a lower metallicity limit for the formation of rocky planets, as heavier elements are less likely to be evaporated by stellar radiation, where the grains survive long enough in the circular disk to clump together and form planets.
Finding planets like TOI-561 b can help narrow down these models, which in turn can help us find older rocky exoplanets.
“Though this particular planet is probably not inhabited today,” Kane said, “it may be a sermon for many rocky worlds not yet discovered around the oldest stars of our galaxy.”
And this can help us in our search for habitable worlds. The Earth is about 4.5 billion years old; the earliest signs of life are thought to be about 3.5 billion years old. And yet vertebrates did not appear on the fossil record until about 500 million years ago, give or take.
It takes time to come to a complex life that we know. So if we want to find life more complex than archaea or microbes, planets that are long-lasting and relatively stable will be, scientists believe, most likely to be hospitable.
So while TOI-561 b would not be a great place to visit, it is yet another clue to help us in our eager search for another life out there in the universe.
The team’s research was presented at the 237th meeting of the American Astronomical Society. It has also been accepted in The Astronomical Journal, and is available on arXiv.