It turns out that planets can live a very long time indeed.
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, whipping around the star on a breakneck 10.5-hour orbit.
Obviously an exoplanet so close to its star isn’t likely to be habitable, even if it is rocky like Earth, Venus and Mars. It would have a temperature of 2,480 Kelvin, tidally locked with a magma ocean 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 around 10 billion years.
That’s more than twice as old as the Solar System, nearly 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 yet discovered,” said astronomer Lauren Weiss of the University of Hawai’i.
“Its existence shows that the universe has been forming 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 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 follow-up observations, astronomers were able to determine the orbital periods and sizes of the three exoplanets.
TOI-561 d, the outermost, is around 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 period of just over 10.5 hours.
The team also conducted radial velocity measurements. As planets orbit a star, that star doesn’t sit still. Each exoplanet exerts its own gravitational tug on the star, resulting in a little complex dance that compresses 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 an exoplanet’s gravitational tugging and 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 Earth’s, about five grams per cubic centimetre.
“This is surprising because you’d 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.”
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 collisions between massive dead stars. Only once stars have died and spread these elements out into space can they be taken up into other objects.
So, the very oldest stars in the Universe are very poor in metals. TOI-561, for instance, is low in metallicity. And any planets that formed in the earlier Universe should likewise have low metallicity.
Previous research has suggested that there is a lower metallicity limit for rocky planet formation, since heavier elements are less likely to be evaporated by stellar radiation, the grains surviving long enough in the circumstellar disc to clump together and form planets.
Finding planets like TOI-561 b can help constrain those models, which in turn could help us locate more ancient rocky exoplanets.
“Though this particular planet is unlikely to be inhabited today,” Kane said, “it may be a harbinger of a many rocky worlds yet to be discovered around our galaxy’s oldest stars.”
And this can aid us in the search for habitable worlds. Earth is around 4.5 billion years old; the earliest signs of life are thought to be about 3.5 billion years old. And yet vertebrates didn’t appear on the fossil record until about 500 million years ago, give or take.
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 think, the most likely to be hospitable.
So while TOI-561 b wouldn’t be a nice place to visit, it constitutes yet another clue that could help us in our avid 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 into The Astronomical Journal, and is available on arXiv.