As planets known as “hot Jupiters” can rotate in surprisingly narrow orbits

A new study has revealed that strange giant planets, known as hot Jupiters orbiting their suns, have been banished to their special paths by nearby planets and stars.

After analyzing the orbits of dozens of hot Jupiters, a team of astronomers has found a way to catch giant planets in the process of approaching their stars. A new analysis, presented Jan. 27 on, blames strange worlds on gravitational impacts from other massive objects orbiting the same starmany of whom destroyed themselves in the process.

“It’s a pretty dramatic way to create your hot Jupiters,” says Malena Rice, an astrophysicist at Yale University.

Hot Jupiters have long been mysterious. They are in orbit very close to their starsrotates in a few days or less, while all the giant planets in our solar system lie at a great distance from the Sun (SN: 05.06.17). To explain the odd planetsastronomers have proposed three main ideas (CH: 11.05.18). Perhaps the hot Jupiters formed next to their stars and remained in place, or perhaps they began to recede further and then slowly turned inward. In any case, the orbits of the planets must have circular orbits aligned with the equator of their stars, because the worlds inherited their paths from the material in the protoplanetary disks that gave birth to them.

However, a new study supports a third idea: gravitational interaction with another giant planet or companion star first throws a planet the size of Jupiter into a very elliptical and inclined orbit that brings it closer to its star. In some cases, the planet even rotates incorrectly around its star, the opposite of how it rotates.

In this scenario, every time an ejected planet is swept past its sun, the star’s gravity deprives the planet of orbital energy. This shortens the orbit, gradually making it more round and less inclined until the planet becomes a hot Jupiter in a small circular orbit rebuilt in the same plane as the equator of the star.

Stars typically orbit the planet in a circle before aligning it, and cold stars rearrange their orbit faster than warm stars. So Rice and her colleagues were looking for a relationship between the shapes and tilt of the orbits of dozens of hot Jupiters orbiting stars of varying temperatures.

Generally speaking, the team found that hot Jupiters around cold stars tend to be in well-aligned circular orbits, while hot Jupiters around warm stars are often in elongated orbits. In other words, many orbits around warm stars have not yet established themselves in their final size and orientation. These orbits still have signs that they were formed by gravitational collisions with neighboring bodies in the system, the team concludes.

This is a “simple, elegant argument,” says David Martin, an astrophysicist at Ohio State University in Columbus who was not involved in the study. “They present evidence in a new way that helps strengthen” the idea that other massive objects in the same solar system produce hot Jupiters. He suspects that this theory probably explains most of these planets.

But this means that countless worlds of giants have faced terrible fates. Some of the planets that threw their brothers close to their stars eventually plunged into the same stars themselves, Rice says. And many other planets in general have been expelled from their solar systems, so today these wayward worlds wander in the deep frost of interstellar space, away from the light of any sun. As planets known as “hot Jupiters” can rotate in surprisingly narrow orbits

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