A new way of discovering planets? Astronomers recognize an exoplanet by seeing its Trojan Belts

Although we’ve found thousands of exoplanets over the last few years, we really only have three methods to find them. The first is to observe a star that dims slightly as a planet passes in front of it (the transit method). The second is to measure a star’s wobble when an orbiting planet imparts a gravitational pull to it (Doppler method). The third is to observe the exoplanet directly. Now a new study in the Astrophysical Journal Letters has a fourth method.

Each of the methods we are currently using has its drawbacks. The transit method only works when an exoplanet’s orbit matches 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 that are far away orbit away from their star. But all of these methods only work for planets orbiting middle-aged stars. These are stars that have long since cleared the dust and debris around them. So while we’ve learned a lot about the types of planetary systems that are out there, we’ve learned less about how young star systems form.

Thanks to radio observatories like ALMA, we have a good look at early debris disks around very young stars. These disks emit a weak radio light that ALMA can see particularly well. One of the things we’ve noticed about a lot of these discs is that they have gaps or bands in them. We believe they are caused by young planets that have carved a path in the debris disk during their growth and evolution. The problem is that we can’t be sure what’s happening. There are other possible explanations, such as turbulence or gravitational resonances within the disk leading to the formation of gaps. The problem is that while we can study the structure of the gaps, telescopes like ALMA cannot resolve an actual planet orbiting within a gap. Even a planet as large as Jupiter is too small to clearly see with direct vision.

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

Jupiter is by far the most massive planet in our solar system and has influenced the orbits of smaller bodies like asteroids over time. One of the clear influences is the asteroid belt, where it has created resonant gaps known as Kirkwood gaps. The other is in the collection of asteroids it has collected in its orbit, known as the Trojans.

Trojan asteroids are small bodies that were accidentally trapped in Jupiter’s Lagrange points. These are regions in Jupiter’s orbit about sixty degrees in front of and behind Jupiter itself. Due to the gravitational dance of Jupiter and the Sun, the Lagrange points are fairly deep gravitational wells, so anything found there tends to stay there. So when Jupiter circles the sun, a group of Trojans march in front of and behind it.

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

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

Relation: Long, Fen et al. “ALMA detection of dust inclusions around Lagrange points in the LkCa 15 disk.” The Letters of the Astrophysical Journal 937.1 (2022): L1.