Cosmic Bullying: How Do Big Stars Snatch Up Planets?

Image: Planetary plunder may be occurring in NGC 3324, a star-forming region in the Carina Nebula.

The article is from the "Dialogue" column and is authorized to be published in the "Expert Voice" column of the "Space" website.

Richard Parker's speech at the School of Astronomy, University of Sheffield

Our Sun is not alone in the Milky Way. The nearest star is just four light years away, and it has its own planets. However, this was not always the case. We often overlook young stars. These young stars are in stellar nurseries, nestling together with their other "siblings".

These star nurseries were so crowded that hundreds or even thousands of stars were packed into a space the size of our solar system. These stars interacted violently with each other through frequent and rapid energy exchanges. After millions of years, these star clusters disappeared, and more and more stars were born in the Milky Way.

The latest research results published in the Monthly Notices of the Royal Astronomical Society show the methods and signals of large stars in stellar nurseries plundering planets.

Planetary systems form around young stars soon after they are born. For 30 years, we have had indirect evidence for this phenomenon: the light from young stars emits unexpected infrared radiation, which usually comes from small dust particles (about 100 cm) orbiting the star. This dust eventually forms planets.

In late 2014, the Atacama Telescope in the Chilean desert captured the first images of a planetary formation region, revolutionizing the study of star and planet formation. The first image and subsequent images taken by the Atacama Telescope were spectacular, with many planet-forming disks already fully formed, like Jupiter.

Figure: High-resolution images of planetary morphology captured by Atacama

Planets form quickly after a star is born, even as it continues to interact with other stars of its own. Because planets form so quickly, they are very susceptible to the effects of their crowded surroundings. They can change their trajectory in a variety of ways.

Wandering Planet

Sometimes, the distances of planets from their stars vary so much that they often change the shape of their orbits—often making them less round (or more odd). Occasionally, a planet escapes the gravitational pull of its star and becomes a rogue planet in a star-forming region, unaffected by any stellar gravity.

A small number of planets will be captured by other stars and become planets of other stars, no longer orbiting the star they were born in. Some planets will skip the stage of wandering planets and be directly plundered by other stars, thus changing their original orbits.

In studying giant planet predation, we found that planets born in the most crowded star-forming regions are very likely to be preyed upon or captured by other stars that are much more massive than our sun. The masses of stars vary widely, and our sun is somewhat unusual in that it is about twice the mass of the average large star in the universe. However, there are still some stars that are more massive, and those OB-type stars are the main sources of light in the Milky Way and other galaxies.

The Beast

Although these massive stars are very bright, their lifespans are much shorter than that of our Sun, with some surviving only a few million years (not billions), so we cannot detect their planets.

However, in 2021, a "beast" exoplanet research project led by researchers from Stockholm University discovered two special planets. One of the planets orbits a star with a mass about ten times that of the sun, at a distance of about 550 times the distance between the earth and the sun, and the other planet orbits a star with a mass of nine times that of the sun, at a distance of about 290 times the distance between the earth and the sun.

The "Beast" star research team discovered that these planets orbit stars in the Sco Cen star-forming region in the Milky Way. Scientists are currently focusing on this area of ​​the Milky Way. Regarding these large planets, scientists believe that they are like the large gas planets in the solar system, but their mass is larger and their scale is far beyond the large planets in the solar system, because just one of the planets is as large as our entire planetary system.

However, massive stars emit large amounts of ultraviolet radiation, which could evaporate the gases necessary for the formation of gaseous planets like Jupiter or Saturn, so what would be the fate of these massive planets?

We already know from previous work that planets can be plundered or captured by other stars in crowded star-forming regions, so we further investigated the intrinsic dynamics of massive stars that capture these planets.

Our latest explanation is that these large planets are derailed by planetary capture, that is, although they are formed after the birth of a star as its planet, they can also be easily captured by other stars. The orbital distances of these planetary systems are often very wide and strange, with the Sun-Earth distance being only the minimum distance, and the shape of their orbits is not circular like the orbits of the planets in the solar system.

Perhaps there are captured planets in our own system, such as the elusive Planet Nine, but Jupiter and other large planets are confirmed to have formed in our own solar system.

Our computer simulations predict the frequency of these systems and the characteristics of the planets' orbits, such as single or binary star formation regions. Future research will reveal more about these planets, but for now, we haven't made any other exciting extraterrestrial science discoveries.

BY: Richard Parker

FY: Autumn

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