Are hot Jupiters lonely, or do they have nearby planetary companions? The answer may depend on how these close-in planets form. A recent discovery adds to the small number of known systems containing a hot Jupiter and an inner planet companion and provides clues to hot Jupiter formation.
Planets form in protoplanetary disks, like the one pictured here around the star TW Hydrae. Interactions between young planets and the gas of the disk can cause planetary migration. [S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO)]
The Origins of Hot Jupiters
Hot Jupiters are among the strangest creatures in the exoplanet zoo. These worlds, which are roughly the mass of Jupiter, orbit extremely close to their host stars, achieving temperatures far hotter than the giant worlds in our solar system.
It’s not yet clear how hot Jupiters come to be. Do they form in place, vacuuming up massive amounts of gas in the blazing heat of their host stars? Do they get their start in cooler regions and then migrate to warmer pastures as they form? Or do they form far from their stars, then get kicked closer by gravitational interactions that dramatically change their orbits? To learn more, researchers are collecting all the information on hot Jupiters that they can — and they’re especially interested in the company that hot Jupiters keep.
Close Companions
So far, astronomers know of only a handful of hot Jupiters that have planetary companions whose orbits lie between the hot Jupiter and the host star. Certain theories of hot Jupiter formation, like high-eccentricity migration, predict that inner planets will be ejected or destroyed when a giant planet migrates from a distant orbit to a close-in one, becoming a hot Jupiter. To understand whether high-eccentricity migration is the main way that hot Jupiters form, as current observations suggest, researchers must understand how common these close-in companions are.
Normalized transit signals from the newly discovered inner planet TOI-1408 c (top) and the previously known hot Jupiter TOI-1408 b (bottom). [Adapted from Korth et al. 2024]
Searching through the TESS light curves, the team found evidence for a planet with that orbital period. They suspect that the automated TESS transit search pipeline missed the planet because of its large transit timing variations. Modeling suggests a mass of 7.6 Earth masses and a radius of 2.22 Earth radii, making TOI-1408 c a mini-Neptune planet.
Far-Out Findings
TOI-1408 b has joined the small but growing ranks of hot Jupiters with low-mass inner planet companions. This finding suggests that TOI-1408 b couldn’t have formed through high-eccentricity migration. Instead, in-situ formation or disk migration must be responsible.
The data analyzed in this work held another surprise: TOI-1408 c might not be the only new member of the TOI-1408 family! Korth’s team used radial velocity modeling to search for other planets and found clear evidence for another object in the system with a period of thousands of days. While more follow-up observations are needed to narrow in on the precise properties of this object, Korth’s team suggests it has a mass of roughly 14.6 Jupiter masses and an orbit lasting nearly 7 years.
Citation
“TOI-1408: Discovery and Photodynamical Modeling of a Small Inner Companion to a Hot Jupiter Revealed by Transit Timing Variations,” Judith Korth et al 2024 ApJL 971 L28. doi:10.3847/2041-8213/ad65fd