It turns out that the planets can really 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 discovered three orbital exoplanets – one of which is a rocky world 1.5 times the size of Earth, orbiting the star. on a 10.5-hour orbit puzzle.
Apparently, an exoplanet that is so close to its star is unlikely to be habitable, even if it is rocky like Earth, Venus and Mars. There would be a temperature of 2480 Kelvin, tidal closed by a magmatic ocean on the constant side of the day.
But the TOI-561 system, the planets and everything else, is one of the oldest ever seen, about 10 billion years old.
This is 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.
“Its existence shows that the universe has formed rocky planets almost since its creation 14 billion years ago.”
The three planets, called TOI-561 b, TOI-561 c and TOI-561 d, were identified by NASA’s TESS space telescope. TESS stares 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 these data and subsequent observations, astronomers were able to determine the orbital periods and sizes 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. A TOI-561 b 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. As the planets orbit a star, that star does not sit still. Each exoplanet exerts its own gravitational pull on the star, resulting in a slightly complex dance that compresses and stretches the star’s light as it moves toward us 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 gravitational pull of an exoplanet and calculate the mass of the exoplanet. From this, researchers estimate that TOI-561b is about three times the mass of the Earth.
But its density is approximately the same as on 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 in line with the idea that the planet is extremely old.”
This is because the heavier 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 in collisions between massive dead stars. Only after the stars die and propagate these elements in space can they be absorbed into other objects.
So, the oldest stars in the universe are very poor in metals. TOI-561, for example, has a low metal content. And all the planets that formed in the earlier universe must also have low metallicity.
Previous research has suggested that there is a lower metallic limit for the formation of rocky planets, as heavier elements are less likely to evaporate from stellar radiation and grains survive long enough in the stellar disk to cluster and to form planets.
Finding planets like the TOI-561 b can help limit these patterns, which in turn can help us find more ancient rocky exoplanets.
“Although this planet is unlikely to be inhabited today,” Kane said, “it could be a harbinger of many rocky worlds yet to be discovered around the oldest stars in our galaxy.”
And this can help us in our search for inhabitable worlds. The earth is about 4.5 billion years old; the earliest signs of life are thought to be around 3.5 billion years old. Yet vertebrates did not appear in fossils until about 500 million years ago, give or take.
Complex life, as we know, 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 will most likely be hospitable, scientists say.
So while TOI-561 b would not be a pleasant place to visit, this is another clue that can help us in our ardent 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 is also accepted in The Astronomical Journal, and is available on arXiv.