Giant planets are up to thousands of times Earth's mass. The ways in which such titanic worlds are born have long sparked heated debates. Now, in a study published Monday in Nature Astronomy, researchers have discovered a distant giant planet they suggest may have formed in a completely different way than our Solar System's largest worlds.
And in a rarity in astronomy, the researchers behind the study were able to see the planet directly, aiding in the ability to watch planets grow in unprecedented detail.
"If our results are correct, it means that we have direct evidence that massive planets can form at absurdly large distances from their star," study lead author Thayne Currie, an astrophysicist at the National Astronomical Observatory of Japan in Hilo, Hawaii, tells Inverse.
How are giant planets formed?
The Solar System's gas giants, Jupiter and Saturn, are respectively about 318 and 95 times Earth's mass. However, gas giants can theoretically grow much larger, up to 13 times Jupiter's mass, before astronomers stop thinking of them as planets and start thinking of them as odd in-between bodies known as brown dwarfs — neither planet nor star.
Previous research suggested Jupiter and Saturn likely formed when rocks clumped together into solid cores whose gravitational pulls then rapidly attracted titanic shells of gas. However, scientists have long suspected that not all gas giants were born via this "core accretion" model.
Jupiter and Saturn orbit roughly five to 10 astronomical units (AU) from the Sun, respectively, with one AU equal to the average distance between Earth and the Sun (93 million miles), and the core accretion model accounts for gas giants born at such areas. However, astronomers have detected a small minority of giant exoplanets more than five times Jupiter's mass orbiting 50 to 200 AU from their stars, where there was likely not enough matter to form them via the core accretion model.
Instead, researchers have suggested that these supermassive gas giants might have formed via a process known as disk instability. In this model, large fragments of the protoplanetary disks of gas and dust surrounding young stars can rapidly and violently collapse under their own gravity to form giant worlds.
One way to find proof that gas giants can form via disk instability is to directly image protoplanets. However, the first and only confirmed protoplanet to date, the gas giant PDS 70b, was discovered near the final stages of its assembly, clearing out a gas-and-dust-depleted space around it. A better way to see if gas giants might form via disk instability is to analyze an infant world during earlier stages of its growth, when it is still embedded within its star's protoplanetary disk.
WHAT DID THE SCIENTISTS DO? — Scientists used the Subaru Telescope at the Mauna Kea Observatory in Hawaii and the Hubble Space Telescope to analyze the young star AB Aurigae, located nearly 510 light-years from Earth in the constellation Auriga, the Charioteer. Roughly 2.4 times the mass of the sun and 60 times brighter, AB Aurigae is only about 1 million to 5 million years old.
Within the star's protoplanetary disk, the researchers discovered what they suggest is a growing Jupiter-size exoplanet, AB Aurigae b, about nine times Jupiter's mass and 2.75 times as wide.
"Nature can surprise you," Currie says. "At best, I originally thought we were going to get a very nice image of AB Aurigae's protoplanetary disk, which would be useful for... something? We did get that — the images are absolutely stunning. Evidence for a protoplanet made the data even more interesting."
However, "the field of detecting protoplanets is a vast graveyard filled with claimed detections that were later refuted or at least disputed," Currie says. To support their claim that AB Aurigae b is a protoplanet and not just some temporary feature of the protoplanetary disk, he notes that it appears to orbit its star, and its light differs from the scattered starlight one would expect to see just from protoplanetary disk material.
"We detected it in 12 different data sets, using five different instruments, and over 13 years," Currie says of AB Aurigae b. "Any work that comes up with an alternate hypothesis will have to do better than that."
"Their patience and meticulousness paid off," Laurent Pueyo, an exoplanet astronomer at the Space Telescope Science Institute in Baltimore who did not take part in this work, tells Inverse. "The detection is unambiguous."
"This is an incredibly exciting discovery of the star and planet formation process in action," Kaitlin Kratter, an astrophysicist at the University of Arizona who was not a part of this work, tells Inverse.
WHAT DID THEY FIND? — The scientists estimated the embryonic world was located a long distance from its star — roughly 93 AU, or about three times Neptune's distance from the Sun.
Spirals of gas seen in the wake of AB Aurigae b's path through its star's protoplanetary disk are of a kind predicted by disk instability models of planetary formation, "an entirely different process from that which was responsible for Jupiter and Saturn in our own Solar System," Currie says. "Nature is quite clever."
"This discovery makes a strong case for the formation of a massive gas giant planet by the heretical gas disk gravitational instability mechanism," Alan Boss, a theoretical astrophysicist at the Carnegie Institution for Science in Washington, D.C., tells Inverse. Although Boss did not participate in this study, he proposed the disk instability model in 1997.
"Given the incredible variety of exoplanets that have been discovered since 1996, the need for more than one specific formation mechanism is obvious," Boss adds.
However, although Kratter believes it is quite plausible that AB Aurigae b formed via disk instability, she suggests that it may not turn out to be a protoplanet after all. Given its apparent current size and how it is likely to draw in even more gas and dust from its star's protoplanetary disk, "I believe they have identified a newly born object that is destined to be a brown dwarf or star."
"Why does the name matter? Star? Brown dwarf? Planet? It matters if we hope to connect observations of formation to the final end products of the star and planet formation process," Kratter says. "Such distinctions matter for making predictions — they can drive instrument and telescope design, and help other scientists target their theoretical and observational efforts in the right areas."
WHAT'S NEXT? — The scientists detected signs that two more protoplanets may orbit AB Aurigae at distances of 430 and 580 AU. Analyzing these potential worlds, perhaps with the James Webb Space Telescope, may shed even more light on disk instability, Currie says.