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Comment by part1of2 | Hacker News Reader

> However, theories of planet formation and evolution predict the existence of free-floating (rogue) planets, gravitationally unattached to any star.

What's the theory that predicted this? After billions of years, I would expect all planets be attached (gravitationally) to a star, shouldn't they?

> Indeed, a few years ago, Polish astronomers from the OGLE team from the Astronomical Observatory of the University of Warsaw provided the first evidence for the existence of such planets in the Milky Way.

Wait. Evidence is not the theory. Did they predict this or discover it?

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I don't quite understand why you're being so pedantic about this. The earliest reference I could find was from 1987, theorizing that proto-planetary objects could be ejected from a planetary disk and that this likely happened in our solar system due to sling-shotting from Jupiter[1]:

> After Jupiter had accreted large amounts of nebular gas, it could have gravitationally scattered the planetesimals remaining nearby into orbits which led to escape from the Solar System.

But this was purely speculative. It wasn't until many years later that the planet was actually observed.

> After billions of years, I would expect all planets be attached (gravitationally) to a star, shouldn't they?

I am not sure what you mean by that. All planets are formed in disks surrounding a star (or possibly a brown dwarf); rogue planets are planets that were able to escape the influence of that star. It might take billions of years before they get close enough to another star in the galaxy to fall into its influence.

[1] https://ntrs.nasa.gov/citations/19870013947

The comment came off stronger than intended. It sounded like lazy journalism that made a claim, but then used a different conclusion. Thank you fr pointing me to the NASA site.

> that were able to escape the influence of that star.

What's the mechanism here? Is it something with greater mass that pulled it into an escape?

>>that were able to escape the influence of that star.

>What's the mechanism here? Is it something with greater mass that pulled it into an escape?

I'm a layman. Take what I say with skepticism, but I think in this case the parent poster was referring to any mechanism that ends up with a planet getting out of the orbit of the star. One such potential mechanism is gravitational slingshotting.

Let's imagine you have a planet orbiting a star. A satellite flies in towards the planet. As the spacecraft passes by the planet, it gets some of the momentum of the planet as the planet is moving around the star. The planet loses that momentum. This increases the satellite's velocity in relation to the sun. The satellite can reach escape velocity and escape the influence of the star.

Edit: alternatively to reading my explanation, you could read Wikipedia's: https://en.wikipedia.org/wiki/Gravity_assist#Explanation

There's an amazing gif of Voyager 2's slingshots. Pay attention to the velocity as it passes planets: https://en.wikipedia.org/wiki/Gravity_assist#/media/File:Ani...

I don't see why a similar thing couldn't happen to planets.

https://www.google.com/search?q=orbital%20simulator

Play with a few orbital simulators. Many are quite simple, but others allow a great deal of flexibility. It's actually quite a tricky business to get more than a few massive bodies into stable orbits -- gravity alone leads to extremely chaotic systems.

Imagine a planet

In the UK there was a recent series on “The Planets” by Brian Cox - one of the things I learnt was how a large has giant had migrated over time from the outer solar system to the inner and back out to the outer. That type of event have caused a slingshot to accelerate a planet like Venus into a highly elliptical orbit, one which might be further accelerated by other planets to a point where it reached escape velocity - especially if combined with large collisions.

This would happen in the same way the voyagers gained speed as it flew past the gas giants in the 80s.

It’s unlikely, but there are a lot of solar systems and a lot of planets.

Predicting the location of a given rogue planet would be like predicting the position if a tractor in a given field in Nebraska. However once you see the tractor is there you can start to predict how it got there.

> After billions of years, I would expect all planets be attached (gravitationally) to a star, shouldn't they?

Planets are formed around stars, but some escape due to gravitational slingshotting etc.. Some of those might be caught again, but why would you think all are?

The down votes are a bit harsh. Theories of planet formation may be familiar to some of us but clearly not everyone, and if you aren’t intimately familiar with orbital mechanics it might not be obvious that rouge planets are expected to be fairly common. We all get to be one of the lucky 10,000 from time to time.

https://xkcd.com/1053/

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