An extraordinary planetary system made headlines in February when astronomers revealed no less than seven planets, all roughly the diameter of Earth, snuggling close together around a small, dim star. The discovery stood out not only because of the cornucopia of possible sister planets to our own, some possibly harboring life, but also because most astronomers had not expected any substantial planets—certainly not ones as large as Earth—to form around such a tiny star.
Conventional wisdom held that at roughly 8% of the Sun’s mass, the salmon-colored star known as TRAPPIST-1 must have coalesced from a relatively tiny disk of gas and dust with too little mass to also give rise to such hefty planets. In fact, astronomers were so certain that stars like TRAPPIST-1 would not have Earth-sized planets in tow that they almost didn’t bother to look, said Amaury Triaud, an astronomer at the University of Cambridge in the United Kingdom who codiscovered the planets. But the field of exoplanets is a field of surprises. And now that these surprising—and seemingly impossible—worlds have been discovered, astronomers are on a mission to explain how they were built.
Planetary Migration in Miniature
In a study submitted for publication in Astronomy and Astrophysics, Chris Ormel, an astronomer at the University of Amsterdam, and his colleagues calculated that the protoplanetary disk that initially formed the TRAPPIST-1 planets likely contained enough gas and dust to fill 11 Earths. At first glance, it might sound reasonable that a disk of that mass churned up seven Earth mass planets, but that initial mass was scattered across a region that’s 10 astronomical units in radius (roughly the distance from the Sun to Saturn), and the planets are all huddled within a region one hundredth the size. There simply wasn’t enough material in this narrow inner region for the planets to have formed in situ.
Might the TRAPPIST-1 planets have instead formed in the outer disk before some mechanism ushered them into their compact orbits? A mechanism known as planetary migration has been used to explain other compact exoplanetary systems (and potentially our own solar system), but it can’t quite explain the TRAPPIST-1 planets. To begin with, forming planets in the icy regions of the outer disk would lead to planets that are, well, icy. But the TRAPPIST-1 planets appear to be rocky. It’s also quite curious that all of the TRAPPIST-1 planets are roughly the same mass, a characteristic that planetary migration can’t explain.
Instead, Ormel and his colleagues have built a scenario that tweaks the migration scenario and, according to outside experts, deftly explains the TRAPPIST-1 system. Ormel’s team does not propose that the planets formed in the outer regions of the disk and then were jostled inward but that centimeter-sized pebbles that served as the building blocks of those planets rushed inward first. “Then something special happened at the ice line,” the distance from the star beyond which any water freezes, Ormel said. There, clouds of newly arrived pebbles warmed by the star lost any attached ice crystals to evaporation and collapsed to become kilometer-sized “planetesimals.”
At this stage in their theorizing, Ormel and his colleagues once again stray from conventional thought. Unlike most models of planetary formation, in which planetesimals then merge together to become full-fledged planets, Ormel suggests that each planetesimal in the TRAPPIST-1 planetary system continued to grow by accreting more and more pebbles. Postulating that this more rapid and efficient growth process dominated in this case provides an explanation for why so much of the mass of the system’s stellar disk ended up in planets.
The latest model also explains why each of the TRAPPIST-1 planets reaches a specific size and then stops swelling. By the time each planet becomes roughly the size of Earth, its gravity interacts with the gaseous disk in such a way that it clears the gas and bits of debris out of the planet’s path, effectively cutting off the growing orb’s food source. At this point the growth of the planet is truncated, resulting in a world that then continues to migrate inward because of other gravitational interactions with the disk. For the TRAPPIST-1 system, this happened seven times, creating the seven Earth-sized worlds huddling close to their tiny star.
“We presented a story from A to Z,” Ormel said. “So we start with very tiny grains that are inherited from the interstellar medium, and we end with a planetary system—if you think about it, this is amazing.” And experts who were not involved in the study agree. “It seems to be a natural outcome,” said Bertram Bitsch, an astronomer at Lund University in Lund, Sweden, who was flabbergasted that the team didn’t have to tweak their initial parameters to arrive at the correct configuration. “So that is very, very cool.”
Already, the discoverers of the TRAPPIST-1 system have noted that it’s an excellent spot in which to search for life because the system is a mere 40 light-years away (a stone’s throw away, astronomically speaking). But a better understanding of the processes that formed these planets will help astronomers further assess the likelihood that life exists on one (or more) of the Earth-sized worlds. If Ormel and his colleagues are correct that these planets formed very close to the ice line, for example, then it’s likely that water was built into the planets themselves, and they might still harbor the life-sustaining liquid today.
To boot, TRAPPIST-1 isn’t the only peculiar or potentially life sustaining planetary system beyond Pluto. Kepler-11, a Sun-like star situated toward the constellation Cygnus, hosts six planets that scale in size from 2 to 4 times the radius of the Earth. “We can see Kepler-11 as a scaled-up version of TRAPPIST-1,” said Ormel, which suggests that his model might also offer insights into how Kepler-11 and other systems like it formed. That could be a boon for planetary science, given that these so-called super-Earths are currently thought to be the most common type of planet in our galaxy.
Harold Levison, an astronomer at Southwest Research Institute in Boulder, Colo., is excited about how the study might turn the telescope around to shed light on our own solar system. Could elements of this new scenario for the TRAPPIST-1 planets also apply to Earth and its familiar rocky neighbors? Perhaps.
“We really can’t figure out, I think, how our own solar system came to be just by studying our own system,” Levison said. “Because it would be like trying to understand how evolution works by just having one species to look at.”
—Shannon Hall (email: [email protected]), Freelance Writer