Uncovering a Gargantuan Exoplanet Orbiting a Tiny Star Alters Planet Formation Theories
The planets discovery holds a mass equivalent to approximately Neptune, yet over 13 times that of Earth, is orbiting an ultracold M-dwarf star named LHS 3154. This star holds a mere nine times less mass than our sun. M-dwarfs are the smallest and most chilly sort of stars.
The study (published in Science on a Thursday) reveals that this enigmatic planet, dubbed LHS 3154b, orbits its star remarkably close, completing a cycle in just 3.7 Earth-days. As a result, it stands alone as the only known planet in a tight orbit around a diminutive, ultracold star with meager mass within the universe.
Suvrath Mahadevan, a co-author of the study and Professor of Astronomy and Astrophysics at Penn State, commented in a statement, "This discovery truly reveals just how much we still have to learn about the universe. We didn't expect something this massive surrounding such a massless star." No known planet exists in orbits around such a small star.
Stars emerge from massive gas and dust clouds, and the remaining matter forms a disk around the star where planets subsequently form. The quantity of material in the star's disk determines the masses of the planets that form around it. The material in the disk heavily depends on the mass of the star.
For example, small M-dwarfs emerge most frequently in our Milky Way galaxy and are typically orbited by small rocky planets rather than gas giants.
"The disk of material around the meager star LHS 3154 likely won't hold sufficient dense mass to create this planet," Mahadevan stated. "However, it's there, so we must now re-evaluate our understanding of how planets and stars form."
Inhabitable Zone
The discovered exoplanet orbits a star 51 light-years away from the sun and was detected by the Habitable Zone Planet Finder (HPF), mounted at the Hobey-Eberly Telescope at the McDonald Observatory in Texas.
Mahadevan leads a team of scientists who built HPF to locate planets found within the habitable zones of small, cool stars. These habitable zones lie at the perfect distance from a star, where a planet can retain water on its surface and potentially support life.
Lesser stars have colder surface temperatures, which means planets can orbit closer to their host stars without losing fragile elements like water from their surfaces. When a planet orbits very close to its host star, the gravitational pull between the two objects causes noticeable oscillations, which HPF can detect using infrared light.
"Imagine the stars are like a fire", Mahadevan explained. "As the fire cools, you have to get closer to it to stay warm. The same goes for planets. When a star is cooler, a planet must orbit closer to stay warm and retain liquid water." If a planet orbits its ultra-cool star very closely, we can detect this through the subtle color shifts in the stellar light, which indicate the planet's gravitational tug.
Planetary Puzzle
Based on models and observations, the team believes that the planet has a dense core requiring more solid material in the planet-forming disk than can be derived from the star, stated Megan Delamer, a co-author of the study and doctoral student in Astronomy at the Penn State University. An abundance of solid matter.
The researchers estimate that the dust quantity in the disk is at least ten times greater than the amounts typically found around low-mass stars.
"Our current theories on planetary formation cannot explain what we see", Delamer stated. "Based on ongoing measurements with HPF and other instruments, objects like this are likely quite rare, so it's truly exciting to discover one."
Several massive planets have been discovered around low-mass stars, such as the 2019-discovered GJ 3512 b. However, they have much longer orbits and do not orbit as closely to their stars.
"Our discovery represents an extreme test case for all existing theories on planetary formation", Mahadevan commented. "That's exactly why we built HPF: to explore how the most common stars in the Milky Way form planets, and to locate these elusive planets."
