TOI-3884b is a very strange planet. At least by Earthling standards.
The gas giant orbits its small star very closely and quickly. It also goes around the star’s poles instead of its equator.
This oddball orbiter 140 light years from Earth has granted astronomers at CU Boulder a rare chance to study a large starspot in great detail for the first time. The spots form when magnetic fields keep heat from reaching the surface of a star.
Catriona Murray
The opportunity to study one comes when the planet eclipses the active spot every 4.5 days, allowing the team to learn about it using spectroscopy.
The data could eventually help astronomers around the globe in a race to learn more about planets outside our solar system. It could also help narrow the search for ones that might harbor life.
That’s because starspots can interfere with a telescope’s very sensitive observations of light as exoplanets—planets that exist beyond the solar system—pass by stars.
“This is one of the first times we’ve been able to directly measure the temperature spectrum of a starspot with such precision,” said Catriona Murray, a postdoctoral researcher in astrophysical and planetary sciences at CU Boulder and the Massachusetts Institute of Technology.
She’s specifically working to unravel the mysteries of exoplanets.
“Hopefully we can apply some of what we learn from this system to other small stars to be able to have more reliable measurements of the planets’ atmospheres around their stars,” Murray said.
Her research into the “extremely large” sun spot was published in The Astrophysical Journal Letters this past spring.
Distorted view
The goal is to use the detailed observations of the starspot to test against astronomers’ models and predictions of the spots.
Studies suggest stellar contamination in exoplanet research is more prevalent and significant than scientists first thought.
In some cases, a cooler starspot can trick an astronomer into thinking a planet has water vapor in its atmosphere when it actually doesn’t.
“This is so pressing because it's hard to trust the amount of water that we're finding in an exoplanet’s atmosphere,” Murray said. “It's also hard to even trust the detection of water.”
Starspots can also distort readings of an exoplanet’s size and temperature.
By learning more about the spots and how they behave, Murray said scientists could eventually get better at filtering this interference out from their observations of exoplanets.
“Hopefully we can get a library of these starspot measurements and then we can learn more about the stars as a whole rather than just our star,” Murray said.
A Neptune-size planet with a clear atmosphere is shown crossing in front of its star in this artist's depiction. Such crossings, or transits, are observed by telescopes to glean information about planets' atmospheres. By analyzing the amount of light blocked by the planet at different wavelengths, researchers can determine which molecules make up the atmosphere. (Credit: NASA/JPL-Caltech)
Zooming in
It would take a scientist 2 billion years to travel to TOI-3884b in a car at 60 miles per hour.
So the starspot study began with a big challenge: Getting some time on the largest and most powerful space telescope ever built.
The James Webb Space Telescope orbits the sun about a million miles from Earth and is giving astronomers the most detailed history of the universe yet. But, on average, just one out of every 10 proposals for observation time on the telescope is accepted.
A panel of astronomers reviews the requests and decides which ones will be awarded the time.
“The last application cycle, they had about 8,000 hours of observation time available and there were over 100,000 hours proposed,” Murray said.
Murray’s proposal rose to the top. In late 2024, she and her research team gave the coordinates and observing details to the scientists who operate the telescope remotely.
The telescope observed TOI-3884b make several passes over the starspot.
“We didn't take a picture of this star, but we have a sense of what it looks like,” said Zach Berta-Thompson, a professor of astrophysics and planetary science at CU Boulder and a co-author of the study. “This is sort of doing an extraordinarily high-resolution mapping of something way far out there in space.”
Murray said if astronomers could see the star up close, it would look kind of bizarre. The spot covers about 20% of the visible stellar disk. It would look cooler and darker in quite a big chunk of the star.
The work continues. Murray and her research team will be working with the starspot data for years to come and testing it against models.
“It’s becoming more important to understand the star that hosts the planet,” she said. “For a long time, we assumed these stars are kind of the background noise, and now we have to actually really understand what they're doing.”