It turns out that planets can indeed live for 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 exoplanets in orbit – one of which is a rocky world 1.5 times the size of Earth. that swings around the star at breakneck speed. 10.5 hour track.
Obviously, an exoplanet this close to its star is unlikely to be habitable, even if it is rocky like Earth, Venus and Mars. It is said to have a temperature of 2,480 Kelvin, tidal closed off with a magma ocean on the permanent day side.
But the TOI-561 system, planets and all, is one of the oldest ever seen, with an estimated age of about 10 billion years.
That’s more than twice the age of the solar system, nearly as old as the universe itself, and proof that rocky exoplanets can remain stable for a long time.
“TOI-561 b is one of the oldest rocky planets discovered to date,” said astronomer Lauren Weiss of the University of Hawai’i.
“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 planet-hunting space telescope, TESS. TESS stares at parts 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.
Based on this data and follow-up observations, astronomers were able to determine the orbital time and size of the three exoplanets.
TOI-561 d, the outermost, is about 2.3 times the size of Earth, with an orbital period of 16.3 days. TOI-561 c is 2.9 times the size of Earth, with an orbital period of 10.8 days. And TOI-561 b is 1.45 times the size of Earth, with an orbital time of just over 10.5 hours.
The team also performed radial speed measurements. While planets revolve around a star, that star does not sit still. Each exoplanet exerts its own gravity on the star, resulting in a small complex dance that squeezes and stretches the star’s light as it moves towards 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 pull of an exoplanet by gravity and calculate the mass of the exoplanet. From this, the researchers calculated that TOI-561b is about three times the mass of the Earth.
But its density is about the same as that of the Earth, about five grams per cubic centimeter.
“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 consistent with the idea that the planet is extremely old.”
That’s because the heavier elements in the Universe – metals heavier than iron – are forged in the hearts of stars, in the supernovae at the end of a massive star’s life and in collisions between massive dead stars. Only when stars have died and these elements have spread into space can they be incorporated into other objects.
The oldest stars in the universe are therefore very poor in metals. For example, TOI-561 has low metallicity. And all the planets that formed in the earlier universe should also 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 evaporate from stellar radiation, the grains survive long enough in the circumstellar disk to clump together and form planets.
Finding planets like TOI-561 b can help narrow down those models, which in turn can help us locate more ancient rocky exoplanets.
“While this particular planet is unlikely to be inhabited today,” said Kane, “it may be a precursor to many rocky worlds yet to be discovered around our galaxy’s oldest stars.”
And this can help us in the search for habitable worlds. The Earth is about 4.5 billion years old; The first signs of life are thought to be about 3.5 billion years old. And yet vertebrates did not appear on the fossil record, give or take, until about 500 million years ago.
Complex life as we know it takes time to emerge. So if we want to find life more complex than archaea or microbes, planets that are long-lived and relatively stable are most likely to be hospitable, according to scientists.
So while TOI-561 b wouldn’t be a fun place to visit, it is yet another clue that could help us in our eager search for other 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 The Astronomical Journal, and is available on arXiv.