Â鶹ÒùÔº

Large exoplanets are more easily detected than small ones. It's axiomatic. While large planets block out more starlight during transits, small planets block out much less, letting them hide in the overpowering glare from their stars. To help detect sub-Jupiter mass exoplanets, astronomers search for the effect these planets can have on their surroundings.

In young solar systems, chaos rules. Collisions are the norm, and countless collisions between planetesimals over millions of years create debris disks around young stars. These disks can have spiral arms, clumps, and other morphological features that can be shaped by the presence of planets.

In recent research, astronomers examined the disk around a young star named TWA 7 and found clues pointing to a sub-Jupiter mass exoplanet existing in a particular location. Using those clues, they were able to locate the planet and directly image it.

The research is titled "Follow-up Exploration of the TWA 7 Planet–Disk System with JWST NIRCam," and in the Astrophysical Journal Letters. The lead author is Katie Crotts of the Space Telescope Science Institute.

"The young M star TWA 7 hosts a bright and near face-on debris disk, which has been imaged from the optical to the submillimeter," the researchers write. "The disk displays multiple complex substructures such as three disk components, a large dust clump, and spiral arms, suggesting the presence of planets to actively sculpt these features."

TWA 7 is a low-mass star about 110 light-years away. It's been observed many times, and when the Spitzer Space Telescope observed it in infrared, it appeared unusually bright. This indicates that the small star is surrounded by a warm debris disk. Subsequent observations with other telescopes, including Hubble, found the disk and identified rings and a faint spiral arm.

In the summer of 2024, astronomers used the JWST to image TWA 7 in greater detail. The JWST found a brightness dip, which indicates a lack of warm dust. It was a strong clue to the presence of a planet. If it was there, it was located in a low-density part of the debris disk near the star.

While promising, astronomers couldn't rule out a background galaxy as the source. Background galaxies often show up in astronomical images and can confuse the results. In order to determine if it was indeed a planet, the JWST observed the star and its disk again a few weeks later.

"C6 is the most promising of the candidates, as it is located at the same position as a potential planet companion detected with MIRI," the authors write. "Additionally, this companion is at the same location where a sub-Jupiter-mass planet is predicted to reside based on the morphology of Ring 2." They also point out that the amount the candidate moved between observations is consistent with the typical orbit of a Saturn-mass planet.

The new exoplanet still needs more observational confirmation, though. "Although follow-up observations are required to confirm the candidate's proper motion and for further characterization, our NIRCam data strongly support the interpretation of C6 as a Saturn-mass planet, thus making it the lowest-mass planet directly imaged to date," the authors write.

This astronomical detective story is similar to another one involving the star Beta Pictoris. Observations of its disk and ring structures also yielded clues to the presence of two exoplanets. Based on those observations, subsequent observations confirmed the presence of a pair of exoplanets.

Together, TWA 7 and Beta Pictoris show how exoplanet locations can be determined by finding and interpreting indirect clues to their presence.

"These results help further confirm the connection between planets and their debris disks and make TWA 7 the perfect system for studying planet–disk interactions and exoplanet evolution," the authors conclude.