Astronomers at MIT, the University of Liège in Belgium, and elsewhere have discovered a huge, fluffy oddball of a planet orbiting a distant star in our Milky Way galaxy. The discovery, reported today in the journal Nature Astronomy, is a promising key to the mystery of how such giant, super-light planets form.
The new planet, named WASP-193b, appears to dwarf Jupiter in size, yet it is a fraction of its density. The scientists found that the gas giant is 50 percent bigger than Jupiter, and about a tenth as dense — an extremely low density, comparable to that of cotton candy.
WASP-193b is the second lightest planet discovered to date, after the smaller, Neptune-like world, Kepler 51d. The new planet’s much larger size, combined with its super-light density, make WASP-193b something of an oddity among the more than 5,400 planets discovered to date.
“To find these giant objects with such a small density is really, really rare,” says lead study author and MIT postdoc Khalid Barkaoui. “There’s a class of planets called puffy Jupiters, and it’s been a mystery for 15 years now as to what they are. And this is an extreme case of that class.”
“We don’t know where to put this planet in all the formation theories we have right now, because it’s an outlier of all of them,” adds co-lead author Francisco Pozuelos, a senior researcher at the Institute of Astrophysics of Andalucia, in Spain. “We cannot explain how this planet was formed, based on classical evolution models. Looking more closely at its atmosphere will allow us to obtain an evolutionary path of this planet.”
The study’s MIT co-authors include Julien de Wit, an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, and MIT postdoc Artem Burdanov, along with collaborators from multiple institutions across Europe.
“An interesting twist”
The new planet was initially spotted by the Wide Angle Search for Planets, or WASP — an international collaboration of academic institutions that together operate two robotic observatories, one in the northern hemisphere and the other in the south. Each observatory uses an array of wide-angle cameras to measure the brightness of thousands of individual stars across the entire sky.
In surveys taken between 2006 and 2008, and again from 2011 to 2012, the WASP-South observatory detected periodic transits, or dips in light, from WASP-193 — a bright, nearby, sun-like star located 1,232 light years from Earth. Astronomers determined that the star’s periodic dips in brightness were consistent with a planet circling the star and blocking its light every 6.25 days. The scientists measured the total amount of light the planet blocked with each transit, which gave them an estimate of the planet’s giant, super-Jupiter size.
The astronomers then looked to pin down the planet’s mass — a measure that would then reveal its density and potentially also clues to its composition. To get a mass estimate, astronomers typically employ radial velocity, a technique in which scientists analyze a star’s spectrum, or various wavelengths of light, as a planet circles the star. A star’s spectrum can be shifted in specific ways depending on whatever is pulling on the star, such as an orbiting planet. The more massive a planet is, and the closer it is to its star, the more its spectrum can shift — a distortion that can give scientists an idea of a planet’s mass.
For WASP-193 b, astronomers obtained additional high-resolution spectra of the star taken by various ground-based telescopes, and attempted to employ radial velocity to calculate the planet’s mass. But they kept coming up empty — precisely because, as it turned out, the planet was far too light to have any detectable pull on its star.
“Typically, big planets are pretty easy to detect because they are usually massive, and lead to a big pull on their star,” de Wit explains. “But what was tricky about this planet was, even though it’s big — huge — its mass and density are so low that it was actually very difficult to detect with just the radial velocity technique. It was an interesting twist.”
“[WASP-193b] is so very light that it took four years to gather data and show that there is a mass signal, but it’s really, really tiny,” Barkaoui says.
“We were initially getting extremely low densities, which were very difficult to believe in the beginning,” Pozuelos adds. “We repeated the process of all the data analysis several times to make sure this was the real density of the planet because this was super rare.”
An inflated world
In the end, the team confirmed that the planet was indeed extremely light. Its mass, they calculated, was about 0.14 that of Jupiter. And its density, derived from its mass, came out to about 0.059 grams per cubic centimeter. Jupiter, in contrast, is about 1.33 grams per cubic centimeter; and Earth is a more substantial 5.51 grams per cubic centimeter. Perhaps the material closest in density to the new, puffy planet is cotton candy, which has a density of about 0.05 grams per cubic centimeter.
“The planet is so light that it’s difficult to think of an analogous, solid-state material,” Barkaoui says. “The reason why it’s close to cotton candy is because both are mostly made of light gases rather than solids. The planet is basically super fluffy.”
The researchers suspect that the new planet is made mostly from hydrogen and helium, like most other gas giants in the galaxy. For WASP-193b, these gases likely form a hugely inflated atmosphere that extends tens of thousands of kilometers farther than Jupiter’s own atmosphere. Exactly how a planet can inflate so far while maintaining a super-light density is a question that no existing theory of planetary formation can yet answer.
To get a better picture of the new fluffy world, the team plans to use a technique de Wit previously developed, to first derive certain properties of the planet’s atmosphere, such as its temperature, composition, and pressure at various depths. These characteristics can then be used to precisely work out the planet’s mass. For now, the team sees WASP-193b as an ideal candidate for follow-up study by observatories such as the James Webb Space Telescope.
“The bigger a planet’s atmosphere, the more light can go through,” de Wit says. “So it’s clear that this planet is one of the best targets we have for studying atmospheric effects. It will be a Rosetta Stone to try and resolve the mystery of puffy Jupiters.”
This research was funded, in part, by consortium universities and the UK’s Science and Technology Facilities Council for WASP; the European Research Council; the Wallonia-Brussels Federation; and the Heising-Simons Foundation, Colin and Leslie Masson, and Peter A. Gilman, supporting Artemis and the other SPECULOOS Telescopes.