Astronomers from the University of Montreal recently took a closer look at the ultra-large exoplanet WASP-107b and found that these so-called 'super-puff' planets can get even weirder than they imagined.

Image Credit: JhonyCoder via Shutterstock / HDR tune by Universal-Sci

WASP-107b is not a newly discovered exoplanet as it was first detected all the way back in 2017, orbiting a star at a distance of approximately 200 light-years away. One peculiar fact about the planet is that it orbits its host star at a distance more than ten times closer than Earth orbits the sun. WASP-107b is referred to as a 'super-puff' because it is incredibly light. The planet is the same size as Jupiter but has only one-tenth of its mass.

To determine the actual mass of the planet, astronomers recently took a closer look using telescopes at the famous Keck observatory in Hawaii. Applying the so-called radial velocity observation technique which allows researchers to establish the mass of an exoplanet by observing its host star's wobbling motion created by the exoplanet's gravitational pull. The observations confirmed that WASP-107b is approximately thirty times as massive as Earth.

The mysterious formation of WASP-107b

Using the Keck data, the team did an analysis to identify what the internal structure of the planet might be. They came to the astonishing conclusion that there must be a solid core that is no more than four times the mass of Earth. This means that over 80% of its mass comes from its dense layer of gas. To put this in perspective, Neptune, a solar system analog when it comes to mass, only derives 10% from its mass from its surrounding gas layer!

W. M. Keck Observatory in Hawaii - Image Credit: Warren Metcalf / HDR tune by Universal-Sci

The fact that WASP-107b is so incredibly light naturally gives rise to additional questions. How is it possible that an exoplanet of such low formed at all? And why it is not losing its enormous layer of gas given the proximity to its host star? Contemporary models for gas-giant formation are founded on the familiar gas-giants found in our own solar system like Saturn. The hypothesis is that a core of at least ten times the mass of Earth is necessary to collect enough gas from the early planet-forming disc surrounding a star.

Professor Eve Lee, a renowned authority on super-puff exoplanets, stated in an interview that there are several hypotheses, the most plausible of which is that the exoplanet began its life further away from its host star. According to Lee, the gas in a planet-forming disc is cold enough that gas buildup can happen a lot faster at more considerable distances. WASP-107b would have migrated towards the center of its star system at a later moment, probably following interactions with other planets or with the planet-forming disc itself.

Yet another exoplanet discovery: WASP-107c

Due to the extra attention that was given to the WASP-107 system, the team discovered another exoplanet was discovered. WASP-107c is a lot larger than 107b, approximately 100 times the mass of Earth.

It seems to be a more conventional gas giant as it is located further out in the system, akin to our own gas giants. A WASP-107c year is approximately three times longer than an Earth year. A huge difference between a 107b year, which takes a little less than six days.

The orbit of WASP-107c is more oval than circular. According to Caroline Piaulet, Ph.D. student at UdeM's Institute for Research on Exoplanets, this eccentric orbit gives a hint of what happened in the system. "Its great eccentricity hints at a rather chaotic past, with interactions between the planets which could have led to significant displacements, like the one suspected for WASP-107b."

There are many more questions about WASP-107b scientists would like to see answered. Previous Hubble observations showed that the planet has only a minimal amount of methane in its atmosphere. This is somewhat puzzling because, according to Piaulet, these types of planets should have vast quantities of the molecule.

Piaulet hopes to be able to do additional research once the James Webb Space Telescope is operational. Who knows what further mysteries will come to light.

The Montreal team published their findings in the Astronomical Journal, together with colleagues from Europe, Japan, and the US. It's listed below, be sure to check it out for a more in-depth look.

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