Sheffield (UK), September 12 (Conversation) Our Sun has a fairly lonely existence in the Milky Way. It sits on its own, four light-years away from the nearest star, with only the company’s planetary system. But it wasn’t always this way. We observe almost exclusively young stars in groups, the so-called stellar nurseries, brushing their shoulders with stellar siblings.
These stellar nurseries are densely populated places where hundreds of thousands of stars are often found in the same volume of space as the Sun alone. Violent interactions, in which stars exchange energy, occur frequently, but not for long. After a few million years, the star clusters dissipated, the Milky Way filled with more stars.
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Our new paper, published in Monthly Notices of the Royal Astronomical Society, shows how massive stars in such stellar nurseries can steal planets away from one another – and what are the signs of such theft.
Almost after the birth of young stars, planetary systems begin to form around them. We’ve had indirect evidence of this for more than 30 years. Observations of light from young stars show an unexpected excess of infrared radiation. This has been (and still is) interpreted as arising from tiny (100th of a centimeter) dust particles orbiting the star in a disk of matter. It is from these dust particles that planets (eventually) are formed.
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The field of star and planet formation underwent a revolution in late 2014 when the first images of planet-forming disks around stars were seen using the Atacama Large Millimeter Array (ALMA) telescope in the Chilean desert. The first and subsequent photos of Alma were amazing. Many of the disks had features and structures that could be attributed to the presence of fully formed Jupiter-like planets.
Planet formation occurs rapidly after star formation begins, certainly while the star is still interacting with its siblings in the stellar nursery. Since planets form so rapidly, they will be affected by the densely populated star-forming environment. The orbits of the planets can change, which can manifest itself in many ways.
Sometimes the planet’s distance from the host star becomes either smaller or larger, but often there is a change in the shape of the orbit – it usually becomes less circular (more “eccentric”). Occasionally, a planet breaks out of orbit around its host star and becomes “free-floating” in a star-forming region, meaning that it is not bound to any star by gravity.
A large portion of the free-forming planets were captured, becoming gravitationally bound to a different star than the one around which they formed. A similar number of planets were stolen from their orbit – they were exchanged directly between the stars without first being afloat.
By studying this great planet theft, we’ve learned that planets that formed in the most populous star-forming regions might be captured or stolen easily by stars much heavier than our sun. Stars are formed with a wide range of masses. Our Sun is somewhat unusual in that it is twice as heavy as the average mass star in the universe. However, relatively few stars are still heavier, and these OB-type stars dominate the light we see in the Milky Way (and other galaxies).
These massive stars are extremely bright but have much shorter lives than the Sun, and in some cases, only live for several million years (instead of billions). Therefore, we may not expect to find planets around them.
However, in 2021, a B-class exoplanet (Beast) abundance study, led by researchers at Stockholm University, discovered a planet orbiting more than 550 times the distance between Earth and the Sun than a star up to ten times the mass of the Sun. and another planet orbiting a star with a mass about 290 times the mass of the sun.
The Beast collaboration found these star-orbiting planets (“monsters”) in the Sco Cen star-forming region, which are currently gradually decaying in the Milky Way. The original explanation given for these animals is that they formed just like the gas giant planets in our solar system, but they are much more massive and farther away because they are an expanded version of our planetary system.
However, massive stars emit copious amounts of ultraviolet radiation, which can evaporate away from the gas that giant planets like Jupiter and Saturn would require to form. So how do the animals end up around them?
We know from our previous work that planet theft and capture can happen in densely populated star-forming regions, and so we looked at our simulations of planets captured or stolen by massive stars.
Our new interpretation of the Beasties is that they ended up in their orbits because planets were stolen – they were born around other stars and were later captured or robbed by massive stars. These planetary systems are usually in wide orbits (at least 100 from the Earth and the Sun), and are very eccentric — quite unlike the nearby circular planets in our solar system, which we think formed there.
There may be a captured planet in our solar system – the hypothetical and elusive Planet 9 – but Jupiter and the other giant planets have formed around our sun.
It also appears that our computer simulations predict the frequency of these systems (one or two per star-forming region), and the orbital characteristics of the animals. Future observations will shed more light on the origin of these planets, but they currently represent another exciting discovery in the field of exoplanet science. (Conversation)
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