Discovery of new exoplanet raises questions about planet formation 

Astronomers have identified one of the youngest exoplanets ever discovered, hidden in the swirl of gas around a newly born star 390 light-years from Earth.

The Jupiter-sized world offers two key opportunities to scientists studying how all planets, including those in our own solar system, develop. A mere 1.5-million-year-old infant compared to its probable lifespan of billions of years, the planet is so young it can still provide clues about its birth. And this study marks the first time astronomers have analyzed an exoplanet’s surrounding disk of gas, which not only provides more information about the planet’s past but also how its future moons will develop.

“The best way to study planet formation is to observe planets while they’re forming,” said Jaehan Bae, the University of Florida professor of astronomy who led the new discovery.

Bae and his international team of collaborators, including UF doctoral student Maria Galloway-Sprietsma, published their findings July 27 in The Astrophysical Journal Letters.

Clues to our past

“I was always curious to learn how our solar system planets had formed in the past,” Bae said. “We can study planets in our solar system directly in many ways. We can get samples of planets, asteroids and comets. But we still can’t see what happened in the past.”

The next best thing to seeing into our own solar system’s history is for scientists like Bae to study the birth of exoplanets, those worlds that orbit stars other than our own sun. So Bae’s team turned to ALMA, a clever array of dozens of radio antennas in the Atacama Desert of northern Chile that is powerful enough to spot these far flung planets. By combining signals from antennas spread across miles of desert, ALMA acts as a single, enormous telescope.

The research group focused on a young star system known as AS 209, one of five stars being studied as a part of a broader ALMA program, known as MAPS, designed to expand out understanding of the chemistry of planet formation.

Scientists can look for clues in each star’s circumstellar disk, the flattened circle of material leftover after the star coalesces in the center of the system. Our solar system once hosted such a disk, and it eventually coalesced into the eight planets.

The AS 209 circumstellar disk has several distinct rings, akin to the rings surrounding Saturn. After analyzing gaps in these rings and other anomalies in the AS 209 disk, the researchers identified the young planet, surrounded by a cloud of material known as a circumplanetary disk.

Because the new study is the first to measure the gas of this surrounding material, it provides a much more complete picture of planet formation than previous studies could accomplish.

“Most of the circumplanetary disk mass is in the gas, not the solid particles. If you see only solid particles in the system, then you’re studying a minor component of the disk,” Bae said. “And in fact one thing we found is the gas-to-dust mass ratio is much, much larger than previously expected, at least 1000-to-1.”

New mysteries

While the planet’s young age and surrounding gas will help astronomers answer existing questions about planet formation, the planet offers up new mysteries of its own.

Namely: How did it form so far away from its own star?

Bae’s team pinpointed the exoplanet at a whopping 200 astronomical units from the AS 209 star. One astronomical unit is the distance between the Earth and the sun. Neptune, the most distant planet, sits at 30 astronomical units, while Pluto orbits roughly 40 astronomical units out from the sun. Beyond that, as far as scientists know, lies nothing but a cloud of small asteroids, comets, and dwarf planets.

Bae’s team has proposed two main models for how the planet formed at this immense distance. In one, the young star’s own gravity jostled the leftover disk of material enough to seed a new planet. The other model relies on the planet seeding itself through the slow accumulation of tiny particles of solid material until there’s enough mass to form a large core.

But neither model sits neatly with the data. The circumstellar disk of the young star seems too small for its gravity to have initiated planet formation at this distance. At the same time, astronomers saw little evidence of the kind of tiny, grain-of-sand-sized particles that clump together to eventually form a planet’s core at the distance of the new exoplanet.

Fortunately, the researchers may not have to wait much longer to get a clearer picture of the planet’s unusual genesis. They have been approved for an early analysis with the new James Webb Space Telescope. The telescope will observe the AS 209 system this month, which will provide key information that could untangle the mystery.

“That’s what makes this system really exciting,” said Bae. “We can follow it with future observations.”