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Comment by btilly | Hacker News Reader

A planet formed out of an accretion disk is generally a planet under continued bombardment from further accretion.

The only exception that we know of is our own Solar System. Where Jupiter acts like a vacuum cleaner to reduce material that might hit Earth. We knkw of no other system with both Earth-like planets and gas giants. Given how catastrophic the remaining asteroids have been for us (bye bye dinosaurs), this is unlikely to be a coincidence.

Unless we find a similar arrangement around a white dwarf, the time spent in the habitable zone isn't the only important factor for stability over evolutionary time frames.

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> The only exception that we know of is our own Solar System. Where Jupiter acts like a vacuum cleaner to reduce material that might hit Earth. We knkw of no other system with both Earth-like planets and gas giants.

Isn't this a current limitation of our exoplanetary detection capabilities rather than something we know about other star systems in general? IIRC exoplanets are most easily discovered by the effect they have on their parent star's brightness when transiting it and/or due to causing the parent star to "wobble". Both those methods would tend to favor finding close and massive planets without necessarily ruling out smaller/more distant ones.

Have exoplanet detection methods improved since I last read about them?

A large enough telescope in space, with a coronagraph could, in teory, find such planets by direct imaging. Right now, I believe only JWST fits the bill, but using it for this purpose would interfere with other types of research, so the coronagraph us used only only to observe planets the wer already discovered by other methods.

I’m pretty sure that new telescope being constructed in Chile will be large enough to directly image planets but I could be wrong.

You're right: https://www.aanda.org/articles/aa/full_html/2021/09/aa41109-...

> Both those methods would tend to favor finding close and massive planets without necessarily ruling out smaller/more distant ones.

They also favor ones whose orbit passes between us and their star, which dims the star and lets us see it. If the planet’s orbit doesn’t block the star, we’ll never know it was there using our current toolset.

I read somewhere that only like 2% of all planet’s have orbits that come between us and their star—which seems like a plausible figure to me, it’s a number you can probably derive from some simple math. This means until we find other ways to observe planets, we are only able to see a very small number of them out there.

Incidentally if other intelligent life is out there planet hunting using the same method of observing planets pass between the star and the observer, it means the odds of them finding earth are pretty small. They’d have to be positioned such that the earth comes between our sun and their line of sight.

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