Massive habitable planet discovered is too big for its star, baffling astronomers

The discovery of a planet that is far too massive for its sun is challenging our existing theories on solar system formation, according to researchers at Penn State.

The experts discovered the planet, which is over 13 times heavier than Earth, circling an “ultracool” star that is notably nine times less massive than the sun.

Unprecedented discovery 

The remarkable finding introduces the heaviest known planet in a close orbit around one of the universe’s smallest and coldest stars, an ultracool dwarf star. This marks the first instance of a massive planet orbiting a star of such low mass.

“This discovery really drives home the point of just how little we know about the universe,” said study co-author Suvrath Mahadevan. “We wouldn’t expect a planet this heavy around such a low-mass star to exist.”

Rethinking planet formation 

Mahadevan elaborated on the conventional understanding of stars and planets’ formation, highlighting the role of remaining gas and dust after a star’s formation in developing planets. 

“The planet-forming disk around the low-mass star LHS 3154 is not expected to have enough solid mass to make this planet,” said Mahadevan. “But it’s out there, so now we need to reexamine our understanding of how planets and stars form.”

Habitable Zone Planet Finder

The detection of LHS 3154b was made possible by the Habitable Zone Planet Finder (HPF), an astronomical spectrograph constructed at Penn State by a team led by Mahadevan. 

This instrument is particularly adept at identifying planets around ultracool stars, where conditions might be favorable for liquid water, a crucial ingredient for life.

LHS 3154 and ultracool stars

“Think about it like the star is a campfire. The more the fire cools down, the closer you’ll need to get to that fire to stay warm,” said Mahadevan. 

“The same is true for planets. If the star is colder, then a planet will need to be closer to that star if it is going to be warm enough to contain liquid water. If a planet has a close enough orbit to its ultracool star, we can detect it by seeing a very subtle change in the color of the star’s spectra or light as it is tugged on by an orbiting planet.”

Hobby-Eberly Telescope

The HPF, located at the Hobby-Eberly Telescope at the McDonald Observatory in Texas, has been instrumental in this discovery, as noted by Guðmundur Stefánsson, NASA Sagan Fellow in Astrophysics at Princeton University and lead author of the paper. 

“Making the discovery with HPF was extra special, as it is a new instrument that we designed, developed and built from the ground-up for the purpose of looking at the uncharted planet population around the lowest mass stars,” said Stefánsson. 

“Now we are reaping the rewards, learning new and unexpected aspects of this exciting population of planets orbiting some of the most nearby stars.”

Exceeding expectations 

The instrument has already yielded critical information in the discovery and confirmation of new planets, Stefánsson explained, but the discovery of the massive planet LHS 3154b exceeded all expectations.

“Based on current survey work with the HPF and other instruments, an object like the one we discovered is likely extremely rare, so detecting it has been really exciting,” said study co-author Megan Delamer, astronomy graduate student at Penn State. “Our current theories of planet formation have trouble accounting for what we’re seeing.”

New insights on LHS 3154

The study suggests that the dust-mass and dust-to-gas ratio in the disks surrounding young stars like LHS 3154 would have to be significantly higher than previously observed to account for the formation of such a massive planet.

“What we have discovered provides an extreme test case for all existing planet formation theories,” Mahadevan said. “This is exactly what we built HPF to do, to discover how the most common stars in our galaxy form planets — and to find those planets.”

More about ultracool dwarf stars

As mentioned above, ultracool dwarf stars are a fascinating and important category of stellar objects in the universe. They are characterized by their relatively low temperatures and small sizes. This detailed report will explore the characteristics, discovery, importance, and challenges related to ultracool dwarf stars.

Characteristics of ultracool dwarf stars

Temperature and Size: Ultracool dwarfs are among the coolest stars, with surface temperatures below approximately 2,700 Kelvin. This temperature range is significantly lower than that of more common stars like the Sun. In terms of size, they are typically smaller than our Sun, often not much larger than the planet Jupiter.

Spectral Classification: These stars are classified at the end of the spectral classification system, which includes L, T, and Y dwarfs. Each class has distinct spectral features: L dwarfs show strong metal hydride absorption and faint or absent titanium oxide, T dwarfs exhibit methane features in their spectra, and Y dwarfs, the coolest of the group, have atmospheres similar to gas giants with water-vapor and ammonia-based clouds.

Luminosity: Ultracool dwarfs are very dim in visible light but are more luminous in the infrared spectrum. This low luminosity makes them challenging to detect with traditional optical telescopes.

Discovery and observation

Detection Methods: Ultracool dwarfs are primarily discovered using infrared surveys, as they emit most of their energy in the infrared range. Space telescopes like the Hubble Space Telescope and the Spitzer Space Telescope have been instrumental in their discovery.

Recent Discoveries: Over the past few decades, numerous ultracool dwarfs have been identified,. Notably, the nearest star system to the Sun, Proxima Centauri, includes an ultracool dwarf. Also, LHS 3154, which we discussed previously in this article, is also an ultracool dwarf star.

Importance in astronomy

Study of Brown Dwarfs and Planetary Mass Objects: Ultracool dwarfs bridge the gap between the hottest planets and the coolest stars. Studying them helps astronomers understand the transition from a star-like object to a planet-like one.

Search for Exoplanets: These stars are excellent targets for the search for exoplanets, especially Earth-sized planets, due to their small size and lower luminosity which makes it easier to detect planets orbiting them.

Challenges and future research

Detection Challenges: Due to their faintness in visible light, ultracool dwarfs are difficult to observe. Advancements in infrared astronomy are crucial for their study.

Atmospheric Studies: Understanding the atmospheres of these stars, which can include complex chemistry and cloud formation, is a key area of ongoing research.

Formation and Evolution: There is ongoing research into how ultracool dwarfs form and evolve over time. This research contributes to the broader understanding of stellar evolution.

In summary, ultracool dwarf stars like LHS 3154, with their unique properties and the challenges they present, continue to be a subject of intense study in astronomy. They offer insights into the complex processes of star and planet formation and are key to our understanding of the diverse nature of celestial objects in our universe.

The study is published in the journal Science.

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