Astronomers have confirmed the existence of exoplanets with extremely small orbits around their stars. But what about exoplanets that come close enough to be swallowed by their star, and what if it is an exoplanet the size of Earth? This is the conclusion of a recent study that AAS Journals hopes to address this problem when an international team of more than 50 researchers studied an Earth-sized exoplanet with an orbital period of just 5.7 hours, known as an “ultra-short period” (USP) exoplanet, which could eventually experience what is known as a tidal disruption that causes it to be swallowed up by its star. This study could help researchers better understand the processes responsible for this, while also further advancing our understanding of the architecture of exoplanets.
Here, Universe today discusses this amazing research with Dr. Fei Dai, an assistant astronomer at the University of Hawaii’s Institute for Astronomy and lead author of the study, about the motivation behind the study, significant findings, possible follow-up studies, the significance of this exoplanet’s Earth size, and whether this could occur in our own solar system. So what was the motivation behind this study?
“Tidal disruption could be a possible fate of rocky planets,” says Dr. Dai Universe todayas he points out in a study published in March 2024 Nature He co-authored and discusses tidal disruption. This study was so profound that it appeared on the cover of the journal. “It appears that about 10 percent of sun-like stars have devoured their rocky planets. This system, TOI-6255, is the best-known precursor to these planet-devouring events. Tidal disruption of rocky planets allows us to study their internal composition and compare it with Earth.”
For the study, researchers analyzed TOI-6255 b, which has a radius about 1.08 that of Earth and a mass about 1.44 that of Earth, and is located just over 20.4 parsecs (65.2 light-years) from Earth. Although Earth’s size makes it promising for life, TOI-6255 b’s 5.7-hour orbit not only makes this exoplanet far too hot for life as we know it, but also means its orbit puts it dangerously close to what’s known as the Roche limit. This is the distance at which a smaller object can orbit a larger object before the larger object’s gravity rips the smaller object to pieces. In addition, TOI-6255 b also experiences the aforementioned tidal disruption that’s common throughout the cosmos, including black holes. So what were the key findings of the study?
Dr. Dai tells Universe today“This planet is doomed to tidal disruption in 400 million years, which is too short on a cosmic scale (~13 billion years). The planet is also distorted by the tides to take on the shape of a soccer ball (10 percent deviation from spherical), in comparison to the Earth’s tidal distortion by the Moon of only 1e-7 (0.0000001) level.”
Regarding possible follow-up studies that the researchers plan to conduct with NASA’s James Webb Space Telescope, Dr. Dai Universe today“Studies of this planet’s orbital phase curve could confirm that it is indeed tidally distorted. We know what the phase curve should look like for a spherical planet; tidally distorted planets show a strong deviation from this. We can also see whether the planet’s surface is covered by lava pools, as would be expected for such a hot planet.”
USPs are exoplanets whose orbits last less than a day and whose mass is less than twice that of Earth. While intriguing, only about 100 USPs have been discovered so far. A 2014 study estimated that about 0.5 percent of them exist around sun-like stars, and a 2019 study examined their overall composition (i.e., the mass of their iron core and mantle). As mentioned, given their extremely short orbits, these worlds are likely too hot for life as we know it, and besides the USPs, there are the well-known “hot Jupiters,” which orbit their stars in just a few days and whose population astronomers estimate to be in the hundreds. As their name literally suggests, these worlds are gas planets the size of Jupiter or larger, and may also be far too hot for life as we know it. But what is the significance of TOI-6255 b being an Earth-sized planet rather than a planet the size of Jupiter or larger?
Dr. Dai tells Universe today“Earth-sized planets are most likely rocky, meaning they consist mainly of an iron core and a silicate mantle. They show us what terrestrial planets in other planetary systems are made of. Jupiter-sized planets are most likely surrounded by a thick hydrogen and helium atmosphere. Life is unlikely to exist on Jupiter-sized planets.”
Although TOI-6255 b has 400 million years to go before it breaks up, watching an exoplanet being torn to pieces by its parent star could provide important insights into the planet’s external and internal composition, helping us better understand the similarities between exoplanets and planets in our own solar system. These unique worlds and their extremely close orbits have challenged our understanding of solar system architecture throughout our entire Milky Way galaxy, as Mercury is the closest planet to our Sun and it still takes 88 days to orbit.
One similarity between our solar system and exoplanet systems is currently the Roche limit. However, the study also focuses on tidal disruption physically distorting TOI-6255 b, with Dr. Dai mentioning above that “tidal disruption could be a potential fate of rocky planets.” So what is the probability that rocky planets in our solar system are affected by tidal disruption, and why?
Dr. Dai tells Universe today“The tidal forces of the planets are minimal in our solar system. However, it is assumed that Saturn’s rings were formed by the tidal forces of the satellites around Saturn. The tidal forces depend strongly on the distance between the orbits; only objects with the shortest orbital periods experience significant tides.”
What new discoveries about tidal forces on Earth-sized planets will astronomers make in the coming years and decades? Only time will tell, and that’s why we do science!
And as always, keep doing science and keep looking up!
