In this age of exoplanet discovery, we’ve discovered thousands of exoplanets of different types. The hot Jupiter is one of the most unusual types. There’s nothing like it in our Solar System.

Hot Jupiters are massive gas planets, and they attract a lot of attention because they’re so close to their stars and reach blistering temperatures. Their existence spawns a lot of questions about their formation and evolution. A new study is trying to answer some of those questions by determining hot Jupiters’ ages.

Hot Jupiters are the most easily-detected exoplanets because they’re so close to their stars and orbit so rapidly. That means they transit often and cause a relatively large dip in starlight when they do. 51 Pegasi b was the first hot Jupiter found, and astronomers spotted it orbiting a Sun-like star in 1995. Now we know of at least 400 hot Jupiters.

Scientists have studied this unusual class of planets and learned a few things. They’re usually tidally locked, and the dayside-nightside temperature difference can reach 1000 Kelvin (726 C) or more. They’ve discovered that hot Jupiters have thermally inverted atmospheres due to the presence of elements like iron, titanium, and vanadium. They’ve also found less water than expected, raising questions about their formation. Hot Jupiters appear to be more common around stars with higher magnitudes like the Sun and less common around low magnitude stars like red dwarfs, although observational biases may play a role there.

How these planets form is a central question in exoplanet science. Do they form like other planets and then migrate towards their star while the star is still young? Or do they fully form further away and migrate later in life in a process astronomers call high-eccentricity migration?

“The question of how these exoplanets form and get to their present orbits is literally the oldest question in our subfield and it is something that thousands of astronomers have been struggling to answer for more than 25 years.”

Kevin Schlaufman, JHU.

A pair of researchers from Johns Hopkins University set out to make some progress on those questions. Their new paper is “Evidence for the Late Arrival of Hot Jupiters in Systems with High Host-star Obliquities.” The lead author is Jacob Hamer, a Ph.D. student in the Department of Physics and Astronomy at Johns Hopkins University. His co-author is Kevin Schlaufman, an assistant professor at JHU who works at the intersection of galactic astronomy and exoplanets. The paper will be published in the Astronomical Journal but is available online at the pre-press site arxiv.org.

This artist’s view shows the hot Jupiter exoplanet 51 Pegasi b, sometimes referred to as Bellerophon, which orbits a star about 50 light-years from Earth in the northern constellation of Pegasus (The Winged Horse). Astronomers found it in 1995, and it was the first hot Jupiter they discovered. Twenty years later this object was also the first exoplanet to be directly detected spectroscopically in visible light. Image Credit: NASA
This artist’s view shows the hot Jupiter exoplanet 51 Pegasi b, sometimes referred to as Bellerophon, which orbits a star about 50 light-years from Earth in the northern constellation of Pegasus (The Winged Horse). Astronomers found it in 1995, and it was the first hot Jupiter they discovered. Twenty years later, this object was also the first exoplanet to be directly detected spectroscopically in visible light. Image Credit: NASA

“The question of how these exoplanets form and get to their present orbits is literally the oldest question in our subfield, and it is something that thousands of astronomers have been struggling to answer for more than 25 years,” said co-author Schlaufman.

The planets in our Solar System have orbits well-aligned with the Sun. They orbit more or less in line with the Sun’s equator, with minor variations. But the population of hot Jupiters contains planets that are aligned with their stars and planets that aren’t. That difference led to difficult questions: Did the two populations of

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