A team of astronomers from the Massachusetts Institute of Technology (MIT) say they have discovered a distant planet that they can’t explain with current planet evolutionary theories.
The newly discovered WASP-193b is a gas giant 50 percent larger than Jupiter, the largest gas giant in our solar system. However, it is only one-tenth as dense, making it the second-most dense exoplanet ever found.
The researchers behind the seemingly impossible find, which also includes experts from Belgium and Spain, say these types of “puffy Jupiters” have left astronomers baffled for over 15 years since they cannot explain how they formed using the best modern-day planet forecasting tools
“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,” explained study 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.”
‘Puffy Jupiter’ is as Dense as Cotton Candy
According to a formal announcement of the discovery, WASP-193b was initially spotted between 2006 and 2008 and then again between 2011 and 2012. Those detections were made by the Wide Angle Search for Planets (WASP) project, which is a collaboration between international astronomers using a pair of robotic observatories in the southern and northern hemispheres.
Together, these robotic arrays of wide-angle cameras scour the stars, looking for the periodic ‘dips’ in light that sometimes indicate a planet is orbiting the star. In the case of WASP 193, a star located 1,232 light years from Earth, the southern WASP observatory spotted these types of dips during both recording sessions.
A detailed analysis of that data determined that a planet was likely crossing in front of its host star every 6.25 days. Further analysis of the amount of light blocked by the planet revealed that it was more or less similar in size to Jupiter. However, when astronomers tried to determine the planet’s mass by measuring how much it affected its host star’s orbit, known as the ‘radial velocity’ method, the numbers didn’t add up. In fact, their initial analysis found no shift in the star’s light spectrum at all, belying the planet’s massive size.
“Typically, big planets are pretty easy to detect because they are usually massive and lead to a big pull on their star,” said study co-author Julien de Wit, an assistant professor in MIT’s Department of Earth. “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.”
After reviewing the data and performing further analysis, the team determined that the planet was actually larger than Jupiter but had to be significantly less dense to exhibit so little gravitational pull on its host star.
“[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,” said lead study author and MIT postdoc Khalid Barkaoui.
The researchers say finding a solid material that has this type of limited density isn’t easy. In fact, in their published study, the authors say the best comparison they could make is that WASP-193b has the same density as cotton candy.
“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.”
While clearly an outlier they cannot easily explain, the researchers say astronomers have seen similar readings in previous exoplanets. Still, those cases are still considered extreme outliers that don’t fit within planetary formation models.
“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.”
Follow-Up Observations Could Unravel the Mystery
In an effort to better understand the planet’s origin, the researchers say they are planning follow-up observations using a technique developed by de Wit. If successful, this process could reveal the planet’s temperature, composition, and even its pressure at various depths of what they suspect is a mostly hydrogen and helium atmosphere.
“Looking more closely at its atmosphere will allow us to obtain an evolutionary path of this planet,” Pozuelos said.
“The bigger a planet’s atmosphere, the more light can go through,” de Wit said in agreement. “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.”
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.