When searching for undiscovered planets in the vast expanse of space, sometimes the best place to look is not up, but into data we already have. A new study led by Peking University undergraduate student Yihan Li, advised by Professor Yifan Zhou of the University of Virginia and Professor Gregory Herczeg at the Kavli Institute of Astronomy and Astrophysics at Peking University, developed a novel method for reusing existing James Webb Space Telescope (JWST) observations to hunt for massive, Jupiter-like planets orbiting in the cold, distant regions of alien solar systems.
Their work focuses on M-dwarfs: small, cool, red stars that make up the most abundant type of star in our Milky Way galaxy. While astronomers have discovered hundreds of small, rocky planets huddled close to M-dwarf stars, a critical question has remained unanswered: Do these systems also harbor giant gas planets orbiting much farther out, in the frigid outer regions analogous to where Saturn orbits in our own solar system? Giant planets on wide orbits act as gravitational architects, using their immense mass to shape the entire structure of a planetary system. Traditional planet-hunting methods struggle with distant giants. The transit method, watching for a star to dim when a planet crosses in front of it, only works if the planet's orbit happens to align with our line of sight, which becomes statistically unlikely for wide orbits. Similarly, the radial velocity method, which detects the gravitational "wobble" a planet induces in its host star, loses sensitivity as planets move farther away and take decades to complete a single orbit.
An illustration of a gas giant planet orbiting its star. Image credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
Direct imaging, literally photographing the planet by blocking out the star's overwhelming glare, offers the best hope. But there's a catch: obtaining deep enough images traditionally requires many hours of precious telescope time on the world's most advanced observatories.
The team, a collaboration between PKU, Virginia, and other international researchers, turned to JWST's growing archive of observations originally collected for an entirely different purpose: studying close-in rocky planets using the transit method. When JWST observes a transiting planet, it must stare at the host star for tens of hours. Several past programs have measured planet atmospheres using long exposures at 15 microns, a wavelength that is also particularly sensitive to the thermal glow of cool objects.
“By mapping the hidden architecture of these alien solar systems, we move one step closer to answering how planets form, and what conditions allowed our own Earth to come to life,” says Yihan Li. “It has been an amazing experience to actually dive into this data and hunt for these hidden worlds.”
The team collected archival MIRI observations of ten M-dwarf systems representing nearly 160 hours of total observation time. By modeling the starlight with mathematical precision and subtracting it from the images, they removed the blinding glare of each central star to reveal the deep background space.
t: This image shows a distant star system after the light of the host star has been subtracted out. There is no visible companion in this area. Right: Detection probability map showing sensitivity across planet mass and orbital semimajor axis parameter space.
While the current study did not reveal any confirmed new planets, which itself offers an interesting hint that wide-orbit giants might be scarce around M-dwarfs, it establishes a powerful new methodology. The images were sensitive enough to detect planets the size of Jupiter, even if they are a frigid -40 degrees (233 Kelvin) and orbiting more than four times further from their star than our Jupiter is from the Sun.
As JWST continues its mission and more transit observations flood the archive, including in the Rocky Worlds DDT program, astronomers will leverage this technique to conduct a substantial survey of wide-orbit gas giants, without requesting a single extra hour of telescope time.
“This was a very creative idea from Prof. Zhou and expertly implemented by Ms. Li,” says Herczeg. “It was wonderful to watch a former PKU undergrad nurture the career of a talented undergrad.”
The paper, “A Search for Wide-orbit Planets Around M-dwarfs using Deep MIRI 15-micron Images” is accepted for publication in an upcoming issue of The Astrophysical Journal and is available at https://arxiv.org/abs/2604.07703.
