New clues about how hot Jupiters form
Since the first hot Jupiter was discovered in 1995, astronomers have been trying to figure out how the red-hot exoplanets formed and ended up in their extreme orbits. Astronomers at Johns Hopkins University have found a way to determine the relative ages of hot Jupiters using new measurements from the Gaia spacecraft, which tracks more than a billion stars.
Lead author Jacob Hamer, a Ph.D. student of Physics and Astronomy, will present the findings this week with press availability June 13 at 1:15 p.m. at the American Astronomical Society conference, which will be streamed live† The work is scheduled to be published in Astronomical Magazine†
Called hot Jupiters because the first one discovered was about the same size and shape as our solar system’s Jupiter. These planets are about 20 times closer to their stars than Earth is to the sun, causing these planets to reach temperatures of thousands of degrees Celsius.
Existing theories of planet formation couldn’t explain these planets, so scientists came up with different ideas about how hot Jupiters might form. Initially, scientists suggested that hot Jupiters could form further away, like Jupiter, and then migrate to their present locations as a result of interactions with their host star’s disk of gas and dust. Or they may form further out and then migrate much later – after the disk has disappeared – through a more violent and extreme process called high eccentricity migration.
“The question of how these exoplanets form and reach their current orbits is literally the oldest question in our subfield, and it’s something that thousands of astronomers have grappled with for more than 25 years,” said study co-author Kevin Schlaufman, an assistant professor works at the intersection of galactic astronomy and exoplanets.
Some hot Jupiters have orbits that are well aligned with their star’s rotation, like the planets in our solar system. Others have orbits misaligned with the equators of their stars. Scientists couldn’t prove whether the different configurations were the product of another formation process, or a single formation process followed by tidal interactions between the planets and the stars. “Without this really accurate method of measuring ages, information was always missing,” says Hamer.
Hamer is one of the first astronomers to use the new data from the Gaia satellite to study the ages of exoplanet systems to find out how they form and evolve. Determining the speeds – the direction velocity – of the stars was key in determining their ages. When stars are born, they move in the same way as each other in the Milky Way. As those stars age, their velocities become more and more different, Hamer said. With this new method, Hamer proved that there are multiple ways hot Jupiters are formed.
“A [formation process] acts quickly and produces coordinated systems, and [the other] occurs over longer timescales and produces misaligned systems,” said Hamer. “My results also suggest that in some systems with less massive host stars, tidal interactions allow the hot Jupiters to realign the axis of their host star’s rotation with their track. †
New data from telescopes on the ground and in space is helping scientists learn more about exoplanets. In April, teams of astronomers, including some from Johns Hopkins, reported findings about the atmospheres of ultra-hot Jupiters made possible by observations from the Hubble Space Telescope.
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