We Found a Gallery of Rogues with Monster-Sized Gas Giants. These are Romantic Facts

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Posted By Sean Raymond, Nuria Miret-Roig & Herve Bouy on Dec 22, 2021

It doesn’t feel right to see a toddler walking down the street by themselves. Toddlers are not quick to wander off the beaten path. If they do, they are swiftly escorted by their parents or grandparents or teachers and returned to where they belong. The majority of planets behave like toddlers. They orbit around stars in a controlled manner. It could be a star with a yellow or red star or a system that has more than one star. We sometimes see planets go rogue, unbound by any star’s gravity, unlike toddlers. This video illustration is from one of us. These “free-floating planets” orbit around stars rather than orbiting around them.

Exoplanets–planets orbiting other stars–are a challenge to find. Most of the exoplanets currently discovered were found by measuring the indirect effects of a planet’s brightness on the star it orbits. Rogue planets, however, don’t need a host star. The odds of a rogue world entering our solar system–like interstellar objects ‘Oumuamua and Borisov–are basically zero, so anyone curious about these interstellar wanderers can’t count on seeing one up close. We don’t want one to be near the planets, as it could cause instability and damage.

The most direct way to detect free-floating planets is simply to take a picture. Three astrophysicists and nine colleagues set their sights on Upper Scorpius. It is a group of nearby baby stars within the constellation. In our new paper, published online in Nature Astronomy, we present the discovery of about 100 free-floating planets there, an unprecedented bounty of rogue planets, roughly doubling the current sample. The stars are not there to block the light of the planets, nor to heat them up and make them shine brightly. The best way to capture planets young is before they cool down and lose their heat. They are visible for a brief time and can be easily detected. That means searching in places where planets are young: in star-forming regions, the birthplaces of stars and planets.

The Upper Scorpius region is so close to our sun that it looks really big, spanning about 170 square degrees–that’s equivalent to an area on the sky with 850 full-moons inside! It was an enormous task to get a good image (pun intended) from this region. As part of the Cosmic-DANCE project, an effort to understand where and how stars form, we compiled more than 80,000 wide-field images of the region taken over the past 20 years with a multitude of different telescopes (adding up to more than 100 terabytes of hard-disk space).

We analyzed the brightness and colors of more than 10 million sources of light in these images. We used the motions and colors of light sources to distinguish the stars in Upper Scorpius (and other galaxies) from the ones in the background due to the size of the area. It is difficult to determine the exact number of free-floating worlds in our sample because it depends on how old the region that formed stars (and by extension each rogue planet).

This is because we can only measure the brightness of each free-floating planet, not its actual mass or size. The planet’s age will determine how long it took to cool down and reach its current brightness. This makes it brighter (and larger) than it was at the time it formed. Objects more massive than about 13 times Jupiter’s mass are brown dwarfs; they are similar to gas giant planets in many ways (including their sizes) but generate heat internally, by nuclear fusion of deuterium. With the brown-dwarf cutoff in mind, the brightest objects in our sample are likely above 13 Jupiter masses, if Upper Scorpius is on the older side, or below 13 Jupiter masses if the region is on the younger side.

With our sample of free-floating planets, we can for the first time address the question of their origins. These objects formed as tiny stars. Decades of study have found that stars generally form according to a pattern in their masses, not-so-poetically called the initial mass function. We tried a number of initial mass functions, but none of them could explain the high number of free-floating stars in Upper Scorpius. We suspect that there must be another mechanism responsible for the large number of rogue planets found.

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The most likely explanation is that many rogue planets formed around other stars in Upper Scorpius but were later ejected into interstellar space. We know that dynamical instabilities in systems of giant planets are basically universal. Computer simulations have shown that each instability causes gravitational scattering between planets. The most likely outcome, as illustrated in the video below, is that one or several planets are ejected completely from the system and cursed to live the rest of their lives with rogues.

Our sample only includes the monsters of the bunch. We are only able to detect planets that are at least four times more massive than Jupiter (which is itself 300 times more massive than Earth). We are almost certain to find a lot of smaller rogues planets. Statistics from the sample of exoplanets suggests that for every four Jupiter-mass gas behemoths, there are 10 or more Neptune-sized planets, and at least as many Earth-sized planets. Upper Scorpius may have thousands of Earth-mass planets. This implies that there is a large population of rogue, rocky planets if you extrapolate to the whole galaxy.

The main challenge for life on a rogue world is keeping warm. To preserve its internal heat, a rogue planet will need a thermal blanket. This is especially important if it requires liquid water. A layer of a few kilometers of ice or ten to 100 bars of molecular hydrogen can provide enough insulation to keep liquid water in place for billions of year. The best hope for life without a star may be on systems of large moons orbiting free-floating gas giants. We know from the Galilean moons around Jupiter that tidal heating–created by repeated flexing by the gas giant’s gravity–can persist for billions of years, and may represent the main heat source on the moon of a free-floating planet. Such a setup is plausible from two different angles: Simulations have shown that the ejection of a gas giant from its home system often leaves its moons intact. If rogue gas giants become mini-stars, they could have their own protoplanetary disks that could contain planets and moons.

Our study is a first peek behind the cosmic curtain at gas giants gone rogue. We hope to discover where these free-floaters come from and what they are doing. With clearer and more detailed observations, we will be able to determine if they are the shrimpiest stars, violent ejecta of the planetary systems, and perhaps even something else.

Sean Raymond is an American astrophysicist working at the Bordeaux Astrophysical Laboratory in France. He also writes a blog at the interface of science and fiction (planetplanet.net), and recently published a book of astronomy poems.

Nuria Miret-Moig is a post-doctoral researcher in astrophysics working in Vienna. She is the lead author on the scientific paper presenting these results.

Herve Bouy is a professor at the Bordeaux Astrophysical Laboratory and the leader of the Cosmic-DANCE project.

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