Orbital decay, where planets eventually fall into their stars and are consumed, is a major aspect of how planetary systems evolve. Before the first exoplanet orbiting a Sun-like star was observed in 1995, astronomers only had the Solar System to inform their models. Since then, surveys by ground-based and space-based telescopes have detected thousands of exoplanets. Thanks to next-generation telescopes like the James Webb Space Telescope (JWST), astronomers can also characterize them.
Among the exoplanets observed, thousands of short- and medium-period planets have been observed around many different types of stars, giving astronomers the chance to study orbital decay. But so far, there have been very few direct detections of exoplanets that support this theory. According to a recent NASA-supported study, the Nancy Grace Roman Space Telescope (RST) will be a game-changer, providing astronomers with many more opportunities to study planets with decaying orbits directly.
The research was led by Kylee Carden, a graduate student in the Department of Astronomy at The Ohio State University (OSU). She was joined by B. Scott Gaudi, the Thomas Jefferson Professor for Discovery and Space Exploration and a University Distinguished Scholar at OSU, and Robert F. Wilson, a postdoctoral fellow at the University of Maryland and NASA’s Goddard Space Flight Center. The study was part of Carden’s graduate work at Ohio State and is currently under review for publication in The Astronomical Journal.
As noted, previous studies have found indirect evidence that planets are consumed in young star systems, ultimately shaping their planetary distribution. This has been noted with Hot Jupiters, which are quite common in the current exoplanet census. These gas giants that orbit closely with their stars have been the subject of immense curiosity to scientists since it was believed that gas giants could only form at greater distances from their stars. As Carden told Universe Today via email, these findings suggest that young systems are shaped by planetary migration:
“First, several studies have found that stars hosting close-in, massive planets (hot Jupiters) are younger than average. This hint could suggest a hot Jupiter destruction mechanism. Second, hot Jupiters are found less frequently around subgiant stars than main sequence stars. Since orbital decay is expected to be more rapid for planets orbiting subgiants, this is another hint that orbital decay could be acting as a destruction mechanism.”
However, direct evidence of this destruction mechanism has been lacking, with only two candidates supporting this theory. These include WASP-12b, a hot Jupiter that orbits so close to its parent star that it is being torn apart, as indicated by its oblong shape, and Kepler 1658b, another hot Jupiter with a very close orbit to its star and a very short orbital period. However, this is expected to change shortly, thanks to the deployment of the RST in 2027, which will conduct a series of Core Community Surveys, including the Galactic Bulge Time Domain Survey (GBTDS).
“The Roman Space Telescope’s GBTDS is going to observe towards the Galactic Bulge, a region dense with stars near the center of our Galaxy,” said Carden. “It has been estimated that Roman will detect ~100,000 transiting planets alone. With all of these planets and an exquisite dataset, we can search for orbital decay, and our baseline estimate is that roughly 5-10 instances of orbital decay will be detectable.”
The GBTDS will leverage Roman’s Wide Field Instrument (WFI, 2.4-meter (7.87 ft) aperture primary mirror and broad near-infrared (NIR) sensitivity to conduct high-precision observations towards the center of the Milky Way. The Transiting Exoplanet Survey Satellite (TESS) and Kepler Space Telescope could detect exoplanets 150 and 2,000 light years from Earth. However, the RST will be sensitive enough to detect planet candidates up to 26,000 light years away. Specifically, the GBTDS will look for microlensing events, which occur when objects come into near-perfect alignment with a background star.
The gravitational force of these objects alters the curvature of spacetime around them, causing light from the background star to become distorted and magnified. These rare alignments act as a “lens,” causing a spike in brightness that alerts astronomers to microlensing events. This will allow the RST to detect exoplanets up to 65,230 light-years away (∼20 kpc) in unexplored regions of the Milky Way. As Carden indicated, this will create a new census of exoplanets that is far more complete:
“Roman will detect exoplanets far outside the Solar neighborhood, showing us what the Galactic population of exoplanets looks like. Roman will illuminate whether orbital decay is a common phenomenon and whether it is typically the ultimate destiny of close-in planets to spiral into their stars. Roman will also help us better understand the physics of tidal dissipation in stars.”
These findings could revolution our current models for how systems form and evolve, including our own! For many years, astronomers have speculated that the early Solar System looked vastly different from what it looks like today. This could also inform astrobiology studies, allowing scientists to learn how planets settle into a star’s habitable zone (HZ), potentially giving rise to life.
Further Reading: arXiv