A new way to discover planets? Astronomers discover exoplanets by seeing the Trojan belts

Although we’ve found thousands of exoplanets in recent years, we really only have three ways to find them. The first is to note that the star dims slightly when a planet passes in front of it (transit mode). The second is to measure the star’s wobble as a planet orbits it giving it gravitational drag (the Doppler method). The third is direct observation of an exoplanet. Now a new study in Astrophysical Journal Letters There is a fourth method.

Each of the methods we currently use has its drawbacks. The transit method works only when the orbit of an exoplanet aligns with our view of the star, the Doppler method tends to favor larger planets orbiting near a small star, and direct observation is best for large planets orbiting far from their star. But all of these methods only work for planets orbiting middle-aged stars. These are the stars that have long since cleared dust and debris around them. So while we’ve learned a lot about the types of planetary systems out there, we’ve learned a little bit about how young star systems form.

Image of HL Tau's planet-forming disk from the Atacama Large Millimeter Array.  Conditions within the disk contribute to the planet's eventual habitability.  Credit: ALMA (ESO/NAOJ/NRAO)
Image of HL Tau’s planet-forming disk from the Atacama Large Millimeter Array. Conditions within the disk contribute to the planet’s eventual habitability. Credit: ALMA (ESO/NAOJ/NRAO)

Thanks to radio observatories like ALMA, we got a good view of early debris disks around very young stars. These discs emit a faint radio glow that ALMA is particularly effective at seeing. One of the things we noticed about many of these discs is that they have gaps or bands inside them. We think they are caused by young planets that paved a path in the debris disk as they grew and evolved. The problem is that we can’t be sure that’s what’s going on. There are other possible explanations, such as turbulence or gravitational resonance within the disc causing the cavities to form. The problem is that while we can study the structure of the gaps, telescopes like ALMA cannot resolve a real planet orbiting inside a gap. Even a planet as large as Jupiter is too small to be clearly detected directly.

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The orbit of Jupiter and the Trojans. Credit: CAS Astronomical Institute / Petr Scheirich

So this new study took a different approach. Instead of trying to spot an exoplanet directly in the disk, why not look for signs inside the debris disk that indicate the presence of the planet? And they’ve found a pattern that works. You can even call it a Trojan horse.

Jupiter is by far the largest planet in our solar system, and over time has influenced the orbits of smaller bodies such as asteroids. One obvious impact is on the asteroid belt, where it caused humming gaps known as Kirkwood gaps. The other is in the group of asteroids it collected in its orbit, known as Trojans.

Trojan asteroids are small objects that are trapped in the Lagrangian points of Jupiter. These regions are located in the orbit of Jupiter about sixty degrees in front of and behind Jupiter. Through the gravitational dance of Jupiter and the Sun, Lagrange points are somewhat deep gravitational wells, so anything that finds itself there tends to stay there. So, as Jupiter parades around the sun, it has a bunch of Trojans running forward and behind it.

A debris disk around the star LkCa 15. Credit: Adam Kraus and Michael Ireland

In this new study, the team focused on a young star known as LkCA 15 and looked for similar gravitational dynamics. By analyzing high-resolution images of the star and its debris disk, they found two very faint clusters of dust. The blocks were in the same orbit within the debris disk, and were separated by an angle of 120 degrees. In other words, the agglomerates have all the signs of being within the Lagrangian points of a small planet. Based on the data, the team estimates the size of the planet roughly the size of Neptune or Saturn. Given that the planet is likely only two million years old, it appears to have formed very quickly.

All this paints an interesting picture of planetary evolution. Large planets seem to form early within a star system, and almost immediately begin to affect their gravitational dance. The next question is whether astronomers can find similar planets in other young star systems using the same method.

Reference: Long, Feng, et al. “ALMA reveals dust trapping around Lagrangian points in a 15 LkCa disk.” Astrophysical Journal Letters 937.1 (2022): L1.

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