How Do Hot Jupiters Get So Close to Their Stars?

Thi illustration of a Hot Jupiter orbiting close to its star. Image Credit: ESA/ATG medialab, CC BY-SA 3.0 IGO

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.

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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

“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 hot Jupiters form differently? Or did they form the same, but one population was subjected to tidal interactions with other planets?

To make any headway on those questions, the pair of researchers needed to know the ages of different hot Jupiters. The team turned to data from the ESA’s Gaia mission to find those ages. Gaia’s mission is ambitious and important: to create a precise 3D map of the Milky Way. “Without this really precise method of measuring ages, there was always missing information,” said Hamer.

Artist’s impression of a hot Jupiter forming in the protoplanetary disk of its parent star. Credit: NASA/JPL/Caltech/R. Hurt

Gaia doesn’t directly measure the ages of objects, but it does measure their positions and velocities. That’s important since younger stars move similarly in their galaxies. But as stars age, that changes, and astronomers know how much it changes. And since planets are only a little younger than their stars, determining a star’s age gives you the planet’s age.

One thing that astronomers know about hot Jupiters is that the systems they’re in are divided into two groups according to the properties of the stars. On the one hand, “… hot Jupiters systems with massive, hot stellar primaries exhibit a wide range of stellar obliquities,” the authors write. “On the other hand, hot Jupiter systems with low-mass, cool primaries often have stellar obliquities close to zero.” Stellar obliquity is the angle between its spin axis and the average orbit of its companion planets. It’s similar to axial tilt in planets. Previous research shows that stellar obliquity constrains a solar system’s evolutionary history.

The authors used the Gaia data and the results of previous research to come up with a new method of determining how hot Jupiters form. They performed Monte Carlo simulations using Gaia data and data from a host of other surveys and missions. Monte Carlo simulations are used in astronomy because they can reliably predict outcomes in systems with a lot of inherent uncertainty.

They discovered that galactic velocity dispersion for stars that host misaligned (>12.8 degrees) hot Jupiters is larger than the galactic velocity dispersion for stars that host aligned (<12.8 degrees) hot Jupiters. Since a larger galactic velocity dispersion means older stars and planets, they concluded that misaligned systems are older than aligned systems. This goes against “… what would be expected if a single formation mechanism produces hot Jupiters with a wide range of stellar obliquities which are then dampened by tides,” they write.

This means two processes form hot Jupiters: a fast one and a slow one.

“One [formation process] occurs quickly and produces aligned 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 to be aligned with their orbit.”

This image from the study shows how different formation processes for hot Jupiters affect the alignment of their velocities and orbits as they age. Image Credit: Jacob Hamer.

The authors also examined the role convective photospheres play in systems hosting hot Jupiters, what role binary stars play, and what role stellar-mass plays.

They concluded that “Together, these results suggest that misaligned hot Jupiters form late relative to aligned hot Jupiters and that in systems hosted by stars with convective envelopes, tidal realignment may subsequently generate aligned systems.”

There’s a lot more astronomers would like to know about these unusual exoplanets and how they form. Like many other topics in astronomy, the James Webb Space Telescope will have something to say about hot Jupiters when it gets going. One particular hot Jupiter is already a target for the JWST.

An artist’s depiction of the hot Jupiter WASP-62b from the perspective of an observer near the planet. WASP-62b has a cloudless atmosphere, making it a prime target for observations with the JWST. Image Credit: M. Weiss / Harvard & Smithsonian Center for Astrophysics.

WASP-62b is a hot Jupiter orbiting a main-sequence star that’s about 1.1 solar masses in the constellation Dorado. It’s misaligned with its star by about 19.5 degrees, so learning more about it will add to the results of this study and build our overall understanding of these curious planets. Previous observations with the Hubble and the Spitzer space telescopes show that WASP-62b has a clear atmosphere, which means that the JWST will be able to identify molecular species in its atmosphere. Understanding the molecular species present can determine the conditions in the protoplanetary disk it formed in.