Orion's mysterious free-floating planets | Hacker News Reader

Orion's mysterious free-floating planets

bbc.com

37 pointsbelter6 comments

The article spends a lot of time painting it as mystery before this seemingly obvious paragraph: "Portegies Zwart favours a different explanation, where Jumbos form in the same way as stars, directly from the collapse of a cloud of gas. Known as in-situ formation, this would require us to rethink how low the density of a gas cloud can be in order to trigger such a collapse. But for Portegies Zwart, "I think in-situ formation is the only one in which I don't have theoretical problems," he says. "It is the most promising."

Why would a nebula not produce _every_ possible size of object over its billions of years?

Maybe this is how stars actually form, gas accretes into chunks of icy matter similar to what happens in Saturn's rings. Then those small objects gravitationally meet and sometimes coalesce into moons, then small planets, big planets, brown dwarves etc. finally when enough stuff has coalesced, the gravity is strong enough to cause fusion and boom you have a star.

Stars usually form through gravitational collapse of gas clouds, not through the kind of bottom up process you are talking about. The physical phenomenon is called the Jeans instability and it only works for gas clouds above a certain density and temperature: https://en.m.wikipedia.org/wiki/Jeans_instability

Smaller things like moons and planets do form in the way you are talking about. There seems to be some limit on how big things can get if they are made this way, and typically we don't think you can get to the point where nuclear fusion starts.

This seems the most straightforward of explanations. I know that a planet tends to loose its heat (and visibility in infrared) quickly after formation -- and might be hard to spot as a rogue planet.

But, the Zwart explanation seems to suggest that the pairs are still potentially drawing stuff in, and might not be the final form that the planet-pairs will take once they exist (or consume) the nebulae. Nothing seems to be stopping these pairs from evolving into stars themselves (aren't binaries far more common than solitary suns, like our own?).

(Purposely terrible writing, but par for the course in popular science.)

More specifically: although it’s certainly plausible there’s a threshold in nebular dynamics for solid object formation because clouds of less total mass fail to congeal, I see no reason a prior why this mass threshold would coincide with the mass necessary to start fusion (i.e., the lightest brown dwarfs). And since we have seen brown dwarfs just above the threshold, but those below it would have been invisible to use before the past ~decade, it seems completely natural to find free-floating planets just below the threshold now that we have acquired the technology to see them.

The objects ranged in size from about half the mass of Jupiter – the largest planet in our Solar System – to 13 times the mass of Jupiter, suggesting they were likely all to be gas giant planets. Jupiter, which is about 11 times the size of the Earth

Comparing two objects by their mass and then immediately switching to using "size" to refer to diameter is such awful science writing. You would never refer to an SUV as being "eleven times the size of a basketball".

For clarity's sake, Jupiter has 1300 times the volume of earth and 318 times the mass.

Also, gas giant sizes don’t grow proportionately to their mass! Mass is a linear measure but diameter is measuring the cube root of volume. Also, heavier gas giants are denser because the added mass compresses the inner layers more.

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