A new study has found that the strange giant planets, known as hot Jupiters, which orbit close to their suns, were followed in their peculiar paths by nearby planets and stars.
After analyzing the orbits of dozens of hot Jupiters, a team of astronomers found a way to capture the giant planets in the process of coming uncomfortably close to their stars. The new analysis, submitted January 27 to arXiv.org, pins the blame for strange worlds on gravitational kicks from other massive objects orbiting the same star, many of which destroyed themselves in the process.
“It’s a very dramatic way of creating your hot Jupiters,” says Malena Rice, an astrophysicist at Yale University.
Hot Jupiters have long been a mystery. They orbit very close to their stars, circling in a few days or less, while all the giant planets in our solar system are located at vast distances from the Sun (SN: 6/5/17) To explain the odd planets, astronomers have proposed three main ideas (SN: 5/11/18) Maybe hot Jupiters formed and stayed next to their stars, or they might have started on the far side and then slowly moved inwards. In any case, 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 disk that gave birth to them.
The new study, however, supports a third idea: gravitational interactions with another giant planet or a companion star throw the first Jupiter-sized planet into a highly elliptical and inclined orbit that brings it closer to its star. In some cases, the planet spins the wrong way around its star, the opposite of the way it spins.
In this scenario, every time a planet overtakes its sun, the star’s gravity robs the planet of its orbital energy. This shrinks the orbit, gradually making it more circular and less inclined, until the planet becomes hot Jupiter on a short, circular orbit, which happens to be in the same plane as the star’s equator. is real.
Stars usually orbit a planet before re-aligning it, and colder stars reorient orbit faster than hotter stars. So Rice and his colleagues looked for a relationship between the size and inclination of the orbits of several dozen hot Jupiters, which revolve around stars of different temperatures.
In general, the team found that hot Jupiters are in well-aligned, circular orbits around cold stars, while hot Jupiters often have long and off-kilter orbits around hot stars. Put another way, many orbits around hot stars have not yet had time to settle into their final shape and orientation. The team concludes that these orbits still contain traces of being shaped by gravitational run-in with neighboring bodies in the system.
It’s a “simple, elegant argument,” says David Martin, an astrophysicist at The Ohio State University in Columbus who was not involved in the study. “They are presenting evidence in a new way that helps to strengthen the idea” that other massive objects in the same solar system generate hot Jupiters. He suspects that this theory probably explains most of these planets.
But it means that innumerable vast worlds have suffered terrible fates. Rice says that some of the planets that brought their brothers closer to their stars drowned in those same stars. And many other planets were completely ejected from their solar systems, so today these misguided worlds wander in the deep freezes of interstellar space, away from any sunlight.