The existence of “hot Jupiters” is one of the oldest mysteries of the era of exoplanet hunting. Hot Jupiters once occupied a significant share of the list of discovered exoplanets, because they are the easiest to detect.
Hot Jupiters are planets about the same mass as Jupiter, but they rotate very close to their stars, usually less than one-tenth the distance at which the Earth revolves around the Sun, that is, even closer to Mercury’s orbit. Hot Jupiters are very different from anything you can see in our solar system.
In the solar system, the total planetary angular momentum is aligned to within 7º of the angular momentum of the Sun’s rotation. This seemed to be a natural consequence of the formation of planets – the Sun and the proto-solar nebula inherited their angular moments from the gas cloud. Therefore, the expectation was that the exoplanets would also have an angular momentum combined with the rotation of their host stars. As it turned out, this is not always the case.
New data from the European Space Agency’s Gaia Space Telescope, which tracks more than a billion stars in the Milky Way, have given a new insight into the formation, evolution and relative age of hot Jupiters. The researchers used measurements of the position and speed of objects to determine the relative ages of the stars. Combining this information with data on the ratio of hot Jupiters to the rotation of their stars has shown that there are two options for the formation of these celestial bodies – fast and slow.
“Without this really precise method of measuring ages, there was always missing information,” Jacob Hamer, a doctoral student in the Johns Hopkins University Department of Physics and Astronomy, said in a statement. Hot Jupiters with orbits offset from the equator of their stars were thought to be formed with a delay relative to those aligned as the planets of our solar system. “One [formation process] occurs quickly and produces aligned systems, and [the other] occurs over longer timescales and produces misaligned systems,” Hamer said in the statement.
The work has been accepted for publication in The Astronomical Journal, and a preprint version is available in the arXiv database.