It turns out that planets can live a very long time.
Around one of the oldest stars in the galaxy, 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 the Earth, whipping around the star on a frantic neck. 10.5 hour orbit.
Obviously, an exoplanet so close to its star is unlikely to be habitable, even if it is rocky like Earth, Venus, and Mars. It would have a temperature of 2480 Kelvin, tidal locked with an ocean of magma on the permanent day side.
But the TOI-561 system, the planets and everything, is one of the oldest ever, at an estimated age of around 10 billion years.
That’s more than twice as old as the solar system, almost as old as the universe itself, and proof that rocky exoplanets can remain stable for a very long time.
“TOI-561b is one of the oldest rocky planets ever discovered,” said astronomer Lauren Weiss of the University of Hawaii.
“Its existence shows that the universe has formed rocky planets almost since its inception 14 billion years ago.”
The three planets, named TOI-561b, TOI-561c, and TOI-561d, were identified by NASA’s planet-hunting space telescope, TESS. TESS looks at sections of the sky, looking for periodic faint dips in the light of distant stars. These are transits, when a planet passes between us and its star.
From this data and the follow-up observations, astronomers were able to determine the orbital periods and sizes of the three exoplanets.
The outermost TOI-561 d is about 2.3 times the size of the Earth, with an orbital period of 16.3 days. TOI-561 is 2.9 times the size of Earth, with an orbital period of 10.8 days. And TOI-561b is 1.45 times the size of Earth, with an orbital period of just over 10.5 hours.
The team also performed radial velocity measurements. When the planets orbit a star, that star does not stand still. Each exoplanet exerts its own gravitational pull on the star, resulting in an intricate little dance that compresses and stretches the star’s light as it moves closer and further 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 gravitational pull of an exoplanet and calculate the mass of the exoplanet. From there, the researchers calculated that TOI-561b is about three times the mass of Earth.
But its density is about the same as the Earth’s, about five grams per cubic centimeter.
“This is surprising because the density is expected to be higher,” said planetary astrophysicist Stephen Kane of the University of California at Riverside. “This is consistent with the idea that the planet is extremely old.”
This is because the heaviest elements in the Universe – metals heavier than iron – are forged in the hearts of stars, in supernovae at the end of a massive star’s life and collisions between stars. massive dead stars. It is only after the stars have died and have spread these elements in space that they can be taken up in other objects.
Thus, the 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 previous Universe are also expected to have low metallicity.
Previous research has 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, with the grains surviving long enough in the circumstellar disc to s’ agglutinate and form planets.
Finding planets like TOI-561b can help constrain these models, which in turn could help us locate older rocky exoplanets.
“While this planet is unlikely to be inhabited today,” Kane said, “it may be a harbinger of many rocky worlds yet to be discovered around the oldest stars in our galaxy.”
And it can help us in the search for habitable worlds. The Earth is about 4.5 billion years old; the first signs of life are believed to date back to around 3.5 billion years ago. And yet, vertebrates only appeared in the fossil record about 500 million years ago, or so.
Complex life as we know it takes time to emerge. So if we are to find life more complex than archaea or microbes, the planets that live long and are relatively stable will be, scientists believe, the most likely to be hospitable.
So while TOI-561 b may not be a pleasant place to visit, it is another clue that could help us in our eager search for another life in the Universe.
The team’s research was presented at the 237th meeting of the American Astronomical Society. It was also accepted in The astronomical journal, and is available on arXiv.