It seems that the planets can really 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 exoplanets in orbit – one of which is a rocky world 1.5 times larger than Earth, tapping around the star on a 10.5-hour orbit break.
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 2,480 Kelvin, blocked by a magmatic ocean on the permanent side of the day.
But the TOI-561 system, planets and all that, 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.
“TOI-561 b is one of the oldest rocky planets ever discovered,” said astronomer Lauren Weiss of the University of Hawaii.
“Its existence shows that the universe forms rocky planets close to 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 space-hunting space telescope, TESS. TESS looks at the sections of the sky, looking for periodic and faint dives 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 periods and orbital dimensions 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 larger than Earth, with an orbital period of 10.8 days. And TOI-561 b is 1.45 times larger than Earth, with an orbital period of just over 10.5 hours.
The team also performed radial velocity measurements. As the planets orbit a star, the star does not sit still. Each exoplanet exerts its own gravitational tug on the star, resulting in a less 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 gravitational pull of an exoplanet and we can calculate the mass of the exoplanet. From this, the researchers calculated that TOI-561b is about three times the mass of 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 notion 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 the life of a massive star, and collisions between dead massive stars. Only once the stars have died and spread these elements in space can they be taken to other objects.
So the oldest stars in the Universe are very poor in metals. TOI-561, for example, has a low level of metallicity. And any planets that formed in the previous Universe should also have low metallicity.
Previous research has suggested that there is a lower limit of metallicity 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 disk to cluster and form planets.
Finding planets like TOI-561 b can help constrain these models, which in turn could help us locate older rocky exoplanets.
“Although this planet is unlikely to be inhabited today,” Kane said, “it may be a harbinger of many rocky worlds that have not yet been discovered around the oldest stars in our galaxy.”
And this can help us in our search for habitable worlds. The earth is about 4.5 billion years old; the first signs of life are believed to be about 3.5 billion years old. And yet, vertebrates did not appear in the fossil record 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 will be, scientists believe, the most likely to be hospitable.
So, while the TOI-561 b would not be a beautiful place to visit, it is still a 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 Astronomical Journaland is available on arXiv.