One of the largest exoplanets to be found orbiting a relatively low-mass star has been discovered, thanks to the way the planet’s gravity drags its star around on its journey through space.
The planet is the fourth world to be spotted in data from the European Space Agency’s Gaia mission, which was designed to map a billion stars in our Milky Way galaxy, cataloguing their masses, luminosities, temperatures and motions through space.
It’s this latter property that led to the discovery of the giant exoplanet Gaia-4b. It orbits a star 244 light-years away and is a super-sized gas giant with a mass 11.8 times greater than that of Jupiter.
“With an orbital period of 570 days, it is a relatively cold gas giant planet,” said Guðmundur Stefánsson of the University of Amsterdam in a statement. This orbital period places it a little closer to its star than Mars is to the sun, but since its star is less massive and less luminous than ours, Gaia-4b is therefore probably not as warm as Mars.
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Yet, by being so massive, Gaia-4b exerts a fierce gravitational pull on its star, an unremarkable orange K-class dwarf with just 64% the mass of our sun. As Gaia plotted the star’s proper motion in the sky, it noticed that the star’s track through space wobbled slightly, as though being pulled to and fro by something in a kind of corkscrew motion across the sky. Astronomers call this technique of detecting exoplanets the astrometric method because it requires the tracking of a star’s position and motion, which is a field known as astrometry.
In fact, Gaia has noticed this wobbling motion for numerous stars, so Stefánsson led a team of astronomers to follow up on these corkscrew systems.
“However, the motion of these stars is not necessarily due to a planet,” said Stefánsson. “Instead, the star might be a pair of stars that are too close together for Gaia to recognize them as separate objects.”
To determine which is the case, Stefánsson’s team employed a trio of instruments: the NEID (pronounced ‘nu-id’) spectrograph on the WIYN 3.5-meter telescope at Kitt Peak National Observatory in Arizona, as well as the Habitable-zone Planet Finder on the 10-meter Hobby Eberly Telescope at McDonald Observatory in Texas and the FIES spectrograph on the 2.6-meter Nordic Optical Telescope on La Palma in the Canary Islands.
These spectrographs are able to measure the radial velocity of a star — how a star wobbles on its axis as it orbits its center of mass shared with an orbiting companion. The greater the mass of the companion, the greater the radial velocity Doppler shift in the star’s light.
Stefánsson’s team had identified 28 candidates in the Gaia data. Of those, 21 were shown to be binary star systems. Five remain inconclusive. One is a star orbited by a brown dwarf — an object that straddles the line between planet and star — now named Gaia-5b, and one turned out to be a planet, Gaia-4b.
Some astronomers might argue, however, that Gaia-4b could also be classed as a brown dwarf; the dividing lines between planet and brown dwarf are murky. Some astronomers define the distinction as depending upon how the object formed. Did it come together through the accretion of rocky material to assemble a giant super-Earth-size core around which humongous amounts of gas wrapped itself, like the formation of Jupiter and Saturn in our solar system? Or did it form like a star, through the gravitational collapse of a gas cloud following the fragmentation of the disk of gas that encircled and fed the main star?
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Other researchers distinguish between planets and brown dwarfs depending upon whether they are able to engage in nuclear fusion reactions of deuterium for a short time (fully fledged stars use hydrogen in their nuclear reactions). But, as Stefánsson’s team note, it’s not really possible to tell what’s going on inside these distant bodies.
A third way is to distinguish between planets and brown dwarfs through their mass. A mass 13 times greater than Jupiter’s is usually considered the lower limit for a brown dwarf, but there’s a certain arbitrariness to this.
Gaia-5b, which orbits a red dwarf star 134 light-years away from Earth, clocks in at 20.9 times the mass of Jupiter, and so it seems safe to call that object a brown dwarf.
Gaia-4b is more troublesome. Purely by mass, it is categorized as a planet. However, Stefánsson’s team looked at the abundance of heavy elements in the central star’s composition, finding its chemistry to be pretty similar to that of our sun. Rocky planets, or the rocky cores of giant planets, need a ready supply of heavy elements in order to form. While our sun and Gaia-4 seem to have enough heavy elements to produce Jupiter-mass planets, it seems questionable that an object 11.8 times the mass of Jupiter could form from the available resources. Stefánsson therefore suggested that this points to Gaia-4b having formed from gravitational collapse following disk fragmentation, where some kind of instability effectively broke the disk of gas around the central star.
Does that mean Gaia-4b should really be called a brown dwarf rather than a planet? If we’re going to call it a planet, then it is one of the most massive planets found around a lower-mass star — such worlds are rare simply because low-mass star systems don’t have lots of extra material available to build giant planets. On the matter of planet or brown dwarf, Stefánsson’s team remain open-minded.
Although Gaia shut down in January following 12 years of observations, there are still reams of data to be analysed from the mission. That data is released in big packets; the most recent was Data Release 3 (DR3) in 2022 (and supplemented with additional data in 2023), and DR4 is expected in 2026.
There are “more planets to come as roughly the last year of data is analyzed,” said Jayadev Rajagopal, a member of Stefánsson’s team at the U.S. National Science Foundation’s NOIRLab, in a different statement. “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.”
The first two planets to be discovered by Gaia, Gaia-1b and Gaia-2b, were both found when Gaia detected them transiting their star. The third planet, Gaia-3b, was a little more complicated in that it was detected astrometrically orbiting one of the stars in a binary system. In total, only around 200 planets have thus far been detected via the astrometric method, but with Gaia DR4 on the horizon, and Stefánsson’s team on the case, they may soon be joined by many more.
The discovery of Gaia-4b and 5b was reported on Feb. 4 in The Astronomical Journal.
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