The process through which big stars steal planets

In the Milky Way galaxy, our solar is quite isolated. It is four light years from the closest star and has just its planetary system for company. But it wasn't always this way. Young stars are nearly always observed in groupings, known as stellar nurseries, where they rub shoulders with stellar siblings. These stellar nurseries are highly crowded locations, with hundreds of thousands of stars frequently sharing the same volume of space as the sun. Violent encounters between stars occur often, but only for a short time. After a few million years, the star clusters disperse, allowing new stars to populate the Milky Way. Our new work, published in the Monthly Notices of the Royal Astronomical Society (opens in new tab), demonstrates how large stars in such stellar nurseries can steal exoplanets away from each other, as well as the signals of such theft.

Planetary systems form almost shortly after young stars are created. For more than 30 years, we have had indirect proof of this. Observations of the light from young stars reveal an unexpected increase of infrared radiation. This was (and still is) described as the result of microscopic dust particles (100th of a centimeter in size) circling the star in a material disc. Planets are (eventually) generated from these dust particles.

ALMA’s high-resolution images of planet-forming discs. (Image credit: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; N. Lira, CC BY-SA)

The study of star and planet formation was transformed in late 2014 when the first photos of planet-forming discs surrounding stars were captured using the Atacama Large Millimetre Array (ALMA) telescope in Chile. Alma's initial and following photographs were nothing short of stunning. Many of the disks featured characteristics and structures that can be ascribed to the presence of fully formed, Jupiter-like planets (opens in new tab). Planet formation occurs quickly after star formation begins, and most likely when the star is still interacting with its siblings in the nursery. Planets will be influenced by the heavily populated star-forming environment since they develop so swiftly. Planets' orbits may be changed, which can manifest in a variety of ways.

Sometimes the planet's distance from the host star decreases or increases, but more frequently than not, the geometry of the orbit changes, generally becoming less circular (more "eccentric"). A planet is occasionally emancipated from its orbit around its host star and becomes "free-floating"in the star-forming area, which means it is not gravitationally tied to any star. A considerable number of free-forming planets are caught and gravitationally linked to a star other than the one in which they originated.

A comparable amount of planets are kidnapped from their orbits and traded straight between stars without first becoming free-floating. We've learnt through analyzing this massive planetary robbery that planets generated in the most crowded star-forming areas may be readily grabbed or taken by stars far heavier than our own sun. Stars develop in a variety of masses. Our sun is a little different in that it is roughly twice the mass of the average star in the cosmos. A tiny number of stars, however, are even heavier, and these "OB-type" stars dominate the light we see in the Milky Way (and other galaxies).

These huge stars are tremendously brilliant, but they have significantly shorter lifetimes than the sun, often only a few million years (rather than billions). As a result, we might not expect to find planets around them. However, in 2021, researchers at the University of Stockholm led the B-star Exoplanet Abundance Study (Beast), which discovered a planet orbiting over 550 times the Earth-sun distance from a star weighing up to ten times the mass of the sun, and another planet orbiting at 290 times the Earth-sun distance around a star nine times the mass of the sun.

These planets ("Beasties") were discovered by the Beast collaboration circling stars in the Sco Cen star-forming area (opens in new tab), which is currently melting into the Milky Way. The initial theory for these Beasties was that they developed similarly to our solar system's gas giant planets, but they are more massive and further off because they are a scaled-up version of our own planetary system. Massive stars, on the other hand, release large amounts of UV light, which can evaporate the gas required to build giant planets like Jupiter and Saturn. So how do Beasties end up in their midst?

We know from past research(opens in new tab) that planet theft and capture can occur in densely populated star-forming areas, therefore we sought for planets caught or taken by large stars in our simulations. Our new theory for the Beasties is that they were born in their orbits as a result of a planetary robbery and were then kidnapped or taken by the huge stars. These planetary systems are often on large (at least 100 Earth-sun) orbits and are very eccentric, in contrast to the round, close-in planets that scientists believe evolved in our solar system.

Perhaps our solar system has a captured planet, the elusive and speculative Planet 9, yet Jupiter and the other huge planets formed around our sun. Our computer calculations also appear to anticipate the number of these systems (one or two per star-forming zone) as well as the Beasties' orbital properties. Future investigations will offer additional information on the planets' origins, but for the time being, they are yet another fascinating finding in the realm of exoplanet study.