What’s the Source of Binary Rogue Planets?

The James Webb Space Telescope (JWST) is already making great strides in helping us to unravel the mysteries of the Universe. Earlier this year, hundreds of rogue planets were discovered in the Orion Nebula. The real surprise to this discovery was that 9% of the planets were paired up in wide binary pairs. To understand how this binary planets formed, astronomers simulated various scenarios for their formation. 

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Hundreds of Free-Floating Planets Found in the Orion Nebula

This image shows the full survey of the inner Orion Nebula and Trapezium Cluster made using the NIRCam instrument on the NASA/ESA/CSA James Webb Space Telescope. This is the long-wavelength colour composite, which focuses on the gas, dust, and molecules in the region with unprecedented sensitivity in the thermal infrared. Credit: NASA, ESA, CSA / Science leads and image processing: M. McCaughrean, S. Pearson.

It appears that rogue planets – free floating worlds that aren’t gravitationally bound to a parent star – might be more common than we thought. New data from the James Webb Space Telescope have revealed 540 (yes, that’s right) planetary-mass objects in the Orion Nebula and Trapezium Cluster.

If confirmed, this would be by far the largest sample of rogue planets ever discovered.

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There Could be Trillions of Rogue Planets Wandering the Milky Way

Artist's rendition of an ice-encrusted, Earth-mass rogue planet free-floating through space. (Credit: NASA’s Goddard Space Flight Center)

A pair of new studies set to be published in The Astronomical Journal examine new discoveries in the field of rogue planets, which are free-floating exoplanets that drift through space unbound by the gravitational tug of a star. They can form within their own solar system and get ejected, or they can form independently, as well. The first study examines only the second discovery of an Earth-mass rogue planet—the first being discovered in September 2020—while the second study examines the potential number of rogue planets that could exist in our Milky Way Galaxy.

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Massive Planet Gone Rogue Discovered

In this artist's conception, a rogue planet drifts through space. Credit: Christine Pulliam (CfA)
In this artist's conception, a rogue planet drifts through space. Credit: Christine Pulliam (CfA)

A massive rogue planet has been discovered in the Beta Pictoris moving group. The planet, called PSO J318.5338-22.8603 (Sorry, I didn’t name it), is over eight times as massive as Jupiter. Because it’s one of the few directly-imaged exoplanets we know of, and is accessible for study by spectroscopy, this massive planet will be extremely important when piecing together the details of planetary formation and evolution.

Most planets outside our solar system are not directly observable. They are discovered when they transit in front of their host star. That’s how the Kepler mission finds exoplanets. After that, their properties are inferred by their gravitational interactions with their star and with any other planets in their system. We can infer a lot, and get quite detailed, but studying planets with spectroscopy is a whole other ball game.

The team of researchers, led by K. Allers of Bucknell University, used the Gemini North telescope, and its Near-Infrared Spectrograph, to find PSO’s  radial and rotational velocities. As reported in a draft study on January 20th, PSO J318.5338-22.8603 (PSO from now on…) was confirmed as a member of the Beta Pictoris moving group, a group of young stars with a known age.

The Beta Pictoris moving group is a group of stars moving through space together. Since they are together, they are understood to be formed at the same time, and to have the same age. Confirming that PSO is a member of this group also confirmed PSO’s age.

Once the age of PSO was known, its identity as a planet was confirmed. Without knowing the age, it’s impossible to rule it out as a brown dwarf, a “failed star” that lacked the mass to ignite fusion.

This new rogue planet is 8.3 + or – 0.5 times the mass of Jupiter, and its temperature is about 1130 K. Spectra from the Gemini scope show that PSO rotates at between 5 to 10.2 hours, and that its radial velocity is within the envelope of values for this group. According to the researchers, determining these properties accurately means that PSO J318.5338-22.8603 is “an important benchmark for studies of young, directly imaged planets.”

PSO is in an intermediate position in terms of other planets in the Beta Pictoris moving group. 51 Eridani-b is another directly imaged planet, only slightly larger than Jupiter, discovered in 2014. The third planet in the group is Beta Pictoris b, which is thought to be almost 11 times as massive as Jupiter.

Beta Pictoris-b in orbit around the debris-disk star Beta Pictoris. Image: ESA/A-M LeGrange et. al.
Beta Pictoris-b in orbit around the debris-disk star Beta Pictoris. Image: ESA/A-M LeGrange et. al.

Rogue, or “free-floating” planets like PSO J318.5338-22.8603 are important because they are not near a star. Light from a star dominates the star’s  surroundings, and makes it difficult to discern much detail in the planets that orbit the star. Now that PSO is confirmed as a planet, rather than a brown dwarf, studying it will add to our knowledge of planetary formation.