Positions of distant super-Earths offer few clues to their origins

March 12, 2021
Super-Earths around distant stars don't tell us much about our own home. (Pixabay/flflflfl)

Super-Earths around distant stars don't tell us much about our own home. (Pixabay/flflflfl)

In faraway solar systems, the distribution of so-called super-Earths may offer surprisingly few hints about the planets' early history, new research suggests.

In a study published March 4 in Monthly Notices of the Royal Astronomical Society, when scientists simulated how these rocky exoplanets outside the solar system form under different conditions, they found that the planets frequently wound up at or near the inner edge of the gas disk that birthed them. 

"The planet formation process completely reshapes the distribution of solids within the disk, so a final planetary system doesn't really have memory of its initial conditions," said Brianna Zawadzki, a doctoral candidate in astronomy at Pennsylvania State University and first author of the paper.

However, her team did observe that super-Earths form very rapidly around low-mass stars, which could indicate how they're able to form and keep an atmosphere.

Understanding a planet's origins can help explain characteristics relating to its orbit, size or composition. 

"When we study the formation process, it gives it that context to fully understand what we observe with the planet," Zawadzki said. For example, "If we observe a planet that has a really circular orbit versus a planet that has a really elliptical orbit, we know that something happened during the formation process to cause those differences."

Since its launch in 2018, NASA's Transiting Exoplanet Survey Satellite (TESS) has systematically scanned the night sky for potential exoplanets. Scientists detect these planets by noting whether an observed star regularly dims, which can mean that an orbiting planet has crossed in front of it and blocked some of its light. 

In a study published March 4 in Science, researchers detailed the discovery of the first Earth-like exoplanet with detectable atmospheric conditions, one whose route periodically crosses TESS, which rarely happens.

But because TESS only stares at each patch of sky for a short period of time, it's more likely to find planets that happen to be close to their star and have brief orbits, Zawadzki says.

"It's kind of difficult with current instrumentation to find really small planets," she added, "because if you only have a small planet, it's not going to block as much starlight and we're only going to see a tiny dip in the star's light or no dip at all."

The TESS mission is expected to discover hundreds of planets around M dwarfs, a very common kind of red dwarf star that is smaller and cooler than this solar system's sun. Zawadzki and her team were particularly interested in rocky super-Earths, which range from two to 10 times the size of Earth. They ran computer simulations that predicted how super-Earths orbiting a star with one-fifth the mass of the sun would form over 100 million years, given various initial conditions.

Planets are born as debris collides and gradually accumulates in rotating disks of dust and gas that surround young stars. Zawadzki and her colleagues began each simulation with 147 same-sized planetary embryos and varied their distance from the star, how densely packed they were and the location of the inner edge of the gaseous disk. Interacting with these gases can push the nascent planets inward, Zawadzki says. She and her team also tested what would happen if gases weren't present when the planets formed.

They found that most of the planet-building collisions took place within the first million years. 

"The planet usually formed entirely within just a few million years, which is really fast for planet formation — faster than we would often see around other types of stars," Zawadzki said. This indicates that the planets would probably finish forming long before the gas disk dissipated, which could mean they're able to form and hold onto an atmosphere by pulling in some of the surrounding gas, she says.

Additionally, embryos that started off closer to their star formed planets more quickly than distant ones did. That's because the closer a forming planet is to its star, the faster it will orbit, giving it more opportunities to come into contact with other bodies, Zawadzki says. 

In the hypothetical event of a missing gas disk, Zawadzki says, the planets might form in different locations than would otherwise be expected.

"This was mostly just as a point of comparison because it's not really practical to think about planet formation happening with no gas disk at all, but we did test what it would be like," she said. "If planets are forming in a gaseous system, then planetary migration is going to cause the system to have only a few close-in planets that are pretty closely spaced, while if we have a system that formed without gas then there is no migration, so you'll have planets that are farther out and separated a little bit more."

Otherwise, the final positions of the planets revealed very little about their origins. 

"If we observe a planetary system and we see three or four planets, it's really tempting to think maybe we can ... look at their masses and get some information on the formation process," Zawadzki said. "But what we found is that it might not actually be possible to do that because everything gets scrambled up so much during the formation processes that you can't really reverse-engineer to find out what the protoplanetary disk looked like."

One limitation of the new findings is that Zawadzki and her team only looked at planets in the inner parts of their solar systems. They also only considered one type of planet and didn't vary the size of the star. 

In the future, the researchers plan to broaden their simulations to solar systems with a wider range of stellar sizes.

The study "Rapid Formation of Super-Earths Around Low-Mass Stars," published March 4 in Monthly Notices of the Royal Astronomical Society, was authored by Brianna Zawadzki and Eric Ford, The Pennsylvania State University; and Daniel Carrera, The Pennsylvania State University and Iowa State University.

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