A Star’s Tiny Wobble Just Revealed a Massive Planet That Shouldn’t Exist

Gaia, a mission designed to map stars, has accidentally become a planet hunter.

Using precise astrometry, it detected stellar wobbles hinting at planetary companions. Follow-up studies confirmed two: a giant exoplanet and a brown dwarf. These findings challenge current planet formation theories and showcase the power of combining different observational techniques.

Gaia’s Unexpected Planet Discoveries

The European Space Agency’s Gaia mission has mapped the positions and motions of stars with extraordinary precision, observing about one billion stars multiple times. This effort has produced a vast 3D map of the Milky Way, which will drive astronomical discoveries for years to come. Gaia’s work is based on astrometry, the precise measurement of celestial objects’ positions and movements.

Though Gaia was designed to study stars, it also detected potential exoplanets. Now, follow-up observations using radial velocity techniques have confirmed one of these planets. This discovery is particularly intriguing because the planet is an outlier in exoplanet science — its characteristics challenge existing models of planet formation.

Gaia wasn’t built to hunt for planets, but its precision made it possible to find them anyway. The planets it detected are massive and orbit low-mass stars. These planets create tiny gravitational tugs on their stars, causing them to wobble slightly — a motion Gaia can measure. However, to confirm these detections, additional observations were necessary.

To validate Gaia’s findings, researchers used the NEID spectrograph on the WIYN 3.5-meter Telescope at the NSF’s Kitt Peak National Observatory. By analyzing the radial velocity shifts caused by these stellar wobbles, they confirmed the presence of a massive exoplanet and a brown dwarf. Their findings, published in The Astronomical Journal, are detailed in a study titled “Gaia-4b and 5b: Radial Velocity Confirmation of Gaia Astrometric Orbital Solutions Reveal a Massive Planet and a Brown Dwarf Orbiting Low-mass Stars.” The study was led by Gudmundur Stefansson from the Anton Pannekoek Institute for Astronomy at the University of Amsterdam.

“Gaia is more than living up to its promise of detecting planetary companions to stars with highly precise astrometry…”

Jayadev Rajagopal, co-author, NSF NOIRLab

Understanding Gaia’s Data and Its Challenges

The most recent Gaia data release contains a list of Gaia AStrometric Objects of Interest (Gaia-ASOIs). They’re stars that appear to be moving as if influenced by an exoplanet.

Lead author Stefansson said, “However, the motion of these stars is not necessarily due to a planet. Instead, the ‘star’ might be a pair of stars that are too close together for Gaia to recognize them as separate objects. The tiny shifts in position that appear to be due to a planet might actually result from the nearly perfect cancellation of the larger shifts in position of the two stars.”

Follow-up spectroscopy can do what Gaia can’t and determine if the objects are binary stars or stars and their orbiting planets. The researchers used the NEID spectrograph and two others—the Habitable-zone Planet Finder and the FIES Spectrograph to perform follow-up observations. In radial velocity, spectrographs measure the blue-shifted and red-shifted light from stars as nearby planets tug on them and make them wobble. It takes extreme precision to do this, and all three spectrographs are capable of it.

Sorting Planetary Candidates from Binary Stars

The researchers examined 28 separate star systems where Gaia detected candidate exoplanets.

According to the results, 21 of the systems have no substellar companions. Instead, these 21 are binary star systems. Five others are inconclusive and require more observations and data before they can be confirmed or refuted.

However, two of the 21 are confirmed: one is an exoplanet now named Gaia-4b, and one is a brown dwarf named Gaia-5b.

Gaia-4b: A Giant Among Exoplanets

Gaia-4b is a massive exoplanet with about 11.8 Jupiter masses. It follows a 571-day orbit around a star with a mass of 0.644 solar masses. It has the distinction of being the first confirmed exoplanet found by Gaia. It’s also one of the most massive planets that have ever been detected orbiting a low-mass star, reflecting the observational bias inherent in Gaia’s method.

“It is an exciting time for both NEID and Gaia,” said Jayadev Rajagopal, a scientist at NSF NOIRLab and a co-author of the paper. “Gaia is more than living up to its promise of detecting planetary companions to stars with highly precise astrometry, and NEID is demonstrating that its long-term radial velocity precision is capable of detecting low-mass planets around those stars. With more candidate planets to come as roughly the last year of data is analyzed, this work is a harbinger of the future where Gaia discoveries of planets and brown dwarfs will need to be confirmed, or rejected, by NEID data.”

Gaia-5b: A Brown Dwarf Discovery

Gaia-5b is a brown dwarf, an object in between planetary mass and stellar mass. Gaia-5b has about 21 Jupiter masses and follows a highly eccentric 358-day orbit around a star with a mass of about 0.34 solar masses.

This study highlights how effective Gaia’s astrometric capabilities are for detecting exoplanets and brown dwarfs. It also exemplifies how different observational techniques—astrometry and radial velocity spectrometry—can work together for more robust results. The combined methods can find a wider range of substellar companion masses and orbital characteristics compared to the transit method, for example.

“If we want to understand how planets are formed, it is necessary to have a vision of how the whole planetary system is composed,” said ESA’s Ana Heras. “Currently, our vision of most systems is only partial because each detection technique is efficient for a certain range of planet sizes and orbital periods. Being able to combine all techniques and data is critical to understand what planetary systems look like and to put our Solar System in context.”

The Future of Gaia’s Exoplanet Discoveries

Gaia-4b is an outlier in exoplanet discoveries. Finding such a massive planet around such a low-mass star is a big test for our planet formation theories. “With respect to stellar host-star mass, the occurrence of massive planets is known to decrease with decreasing stellar mass,” the authors write in their paper. “This has been connected to the fact that less massive stars tend to have less massive protoplanetary disks.” If Gaia and the NEID spectrograph and other facilities can find and confirm more of these massive planets, maybe researchers can make progress in understanding how they form.

Astronomers expect to find more massive exoplanets and brown dwarfs in Gaia data and confirm some of them with spectrographs like NEID. Due to Gaia’s observational method, there will likely be more “outliers” in the data. These outliers are needed to help us understand planet formation and solar system architecture.

“These detections represent the tip of the iceberg of the planet and brown dwarf yield expected with Gaia in the immediate future, enabling key insights into the masses and orbital architectures of numerous massive planets at intermediate orbital periods,” the authors conclude.

Adapted from an article originally published on Universe Today.

Explore Further:

• Gaia Just Found Two Massive Objects That Shouldn’t Exist
• Scientists Just Found a Hidden Planet That’s Absolutely Massive

Reference: “Gaia-4b and 5b: Radial Velocity Confirmation of Gaia Astrometric Orbital Solutions Reveal a Massive Planet and a Brown Dwarf Orbiting Low-mass Stars” by Gudmundur Stefánsson, Suvrath Mahadevan, Joshua N. Winn, Marcus L. Marcussen, Shubham Kanodia, Simon Albrecht, Evan Fitzmaurice, Onė Mikulskytė, Caleb I. Cañas, Juan I. Espinoza-Retamal, Yiri Zwart, Daniel M. Krolikowski, Andrew Hotnisky, Paul Robertson, Jaime A. Alvarado-Montes, Chad F. Bender, Cullen H. Blake, J. R. Callingham, William D. Cochran, Megan Delamer, Scott A. Diddams, Jiayin Dong, Rachel B. Fernandes, Mark R. Giovinazzi, Samuel Halverson, Jessica Libby-Roberts, Sarah E. Logsdon, Michael W. McElwain, Joe P. Ninan, Jayadev Rajagopal, Varghese Reji, Arpita Roy, Christian Schwab and Jason T. Wright, 4 February 2025, The Astronomical Journal.
DOI: 10.3847/1538-3881/ada9e1

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