12 Times Jupiter’s Mass? The Mystery of Gaia-4b and Its Strange Orbit

The hunt for exoplanets has led scientists to uncover some of the most bizarre and fascinating worlds beyond our solar system. Among them, Gaia-4b stands out—not only for its massive size but for the unique method used to confirm its existence. This planet, one of the most massive ever found orbiting a low-mass star, is rewriting our understanding of how planetary systems form.

What Is Gaia-4b? A Giant Among Planets

Gaia-4b is an exoplanet located 244 light-years away, orbiting a low-mass star called Gaia-4. What makes this planet so intriguing is its sheer size—it is 12 times the mass of Jupiter, making it one of the most massive exoplanets ever detected around a star of such low mass.

But it’s not just its mass that’s surprising. Unlike many known exoplanets, Gaia-4b orbits its star at a relatively large distance, completing a full orbit every 570 days. This makes it a cold gas giant, vastly different from the scorching “hot Jupiters” that are often found orbiting close to their stars.

So, how was this unusual planet discovered? The answer lies in a groundbreaking technique that could change the way we detect exoplanets.

How Was Gaia-4b Discovered? The Astrometric Technique

Most exoplanets are discovered using two main methods: the transit method (where a planet passes in front of its star, causing a dip in brightness) and the radial velocity method (which detects the gravitational “wobble” of a star caused by an orbiting planet).

However, Gaia-4b was found using astrometry—a technique that tracks the tiny movements of a star in response to a planet’s gravitational pull. The European Space Agency’s Gaia spacecraft is revolutionizing astronomy by using this method, mapping the positions and motions of billions of stars with unprecedented accuracy.

Why Is Astrometry So Important?

• Unlike the transit or radial velocity methods, astrometry allows scientists to detect planets that have longer orbits—ones that might be missed by other techniques.
• It helps determine the precise mass of an exoplanet, something that is difficult with other methods.
• It enables scientists to detect planets around all types of stars, including those that are faint or difficult to observe with other instruments.

The Role of NEID and Other Ground-Based Observatories

Even though Gaia’s data suggested the presence of Gaia-4b, it wasn’t enough to confirm its existence. Scientists needed follow-up observations to rule out false positives—cases where a detected motion might actually be caused by a binary star system rather than a planet.

To confirm Gaia-4b, astronomers turned to ground-based telescopes equipped with advanced radial velocity spectrographs, including:

• NEID (mounted on the WIYN 3.5-meter Telescope in Arizona)
• The Habitable-zone Planet Finder (HPF) in Texas
• The FIES Spectrograph in the Canary Islands

By combining data from these observatories, scientists were able to measure the planet’s gravitational effect on its host star, confirming that Gaia-4b is indeed a giant planet, not a companion star.

Why Is This Discovery Important?

The confirmation of Gaia-4b is a game-changer for exoplanet research for several reasons:

1. Proving the Power of Astrometry

Gaia-4b is the first exoplanet ever confirmed using the astrometric method. This means that scientists can now use this technique to discover thousands of other exoplanets, including ones that are currently undetectable using traditional methods.

2. Expanding Our Understanding of Planet Formation

Gas giants like Jupiter are usually thought to form far from their stars, where icy materials can accumulate. The discovery of a massive planet like Gaia-4b around a low-mass star challenges traditional models of planetary formation. It suggests that massive planets might form in more diverse environments than previously thought.

3. Finding More Planets Around Low-Mass Stars

Low-mass stars are the most common type of star in the universe, yet they rarely host giant planets. This discovery suggests that there may be more gas giants around small stars than we initially believed.

What’s Next? The Future of Exoplanet Discovery

With Gaia’s astrometric technique now proven, scientists are eager to find more planets using this method. The latest Gaia Data Release has already identified dozens of candidate exoplanets that now require further confirmation.

Exciting Prospects for Astronomy

• Detecting Earth-like planets: Astrometry can detect planets in habitable zones, where conditions might support life.
• Understanding planetary migration: Gaia-4b’s distant orbit challenges models of how gas giants form. Further studies may reveal how planets move over time.
• Enhancing ground-based follow-ups: Instruments like NEID and upcoming missions like the Nancy Grace Roman Space Telescope will refine our understanding of these planets.

Conclusion: A New Era in Exoplanet Discovery

The discovery of Gaia-4b is more than just another exoplanet find—it proves that astrometry is a powerful tool that will shape the future of astronomy. With Gaia continuing its mission, and ground-based observatories working in tandem, we are on the brink of uncovering thousands of new worlds, some of which may one day change our understanding of life beyond Earth.

Reference:

Stefánsson, G. et al. “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 Astronomical Journal, 2025. DOI: 10.3847/1538-3881/ada9e1