This is something that always appears weird to me when reading about exo-planets. They usually have these absurd quick orbits around their star, like a couple of days or two weeks at most. Why is that?
In: Planetary Science
It’s much easier to detect exo-planets that orbit close to their star (and thus have quick orbits) because of the methods used to find them. One method involves watching for planets which pass between their star and earth, which is more likely with a close, fast orbit. And another method uses the wobble produced by the planet’s gravitational effect on the star which is again more likely with closer orbits. We haven’t been looking at any of these stars for 30 years continuously (which is the orbital period of saturn, for example) so we can’t confirm orbital effects of planets with that orbital period
It’s a loaded question, so let me modify it for you in such a way that it should partially answer itself:
“Why do the exoplanets WE FOUND have quick orbits”.
The answer to this proper question is that in 99.9% of cases we never actually see the exoplanet. The star has to be particularly close and the planet particularly big to get a shot at that (and even then only “technically”, as in we can get a spectrum of light passing through the atmosphere).
We see dimming or wobbling patterns of a star. And from those patterns, astrophysicists estimate the potential for it being an exoplanet and it’s likely properties.
But to determine whether there’s an actual pattern there, and not just random noise or a statistical fluke, you need a lot of data. And to get a lot of data, you need to be able to collect it in a reasonable time. And for a scientist, that means repeated transitions, as many as possible, within their observation timeframe, which usually isn’t that long.
So they observe a lot of fast orbits because that’s the one they can actually discern. It’s not impossible to at least attempt to measure the effect of a long orbit on a star, but it is particularly difficult. They just go undetected.
My layman’s guess is that not all exoplanets have such fast orbits, but the ones with fast orbits are easier to identify and verify. Think about it this way: Neptune orbits around the sun once every 164 Earth-years. For an alien in a distant star system to spot Neptune, they would have to be in a star system that aligns with Neptune’s plane of orbit, *and* be mapping the Sun’s light output in that very brief window of time where Neptune obscures the sun. Even then, that wouldn’t be confirmation as from the alien’s perspective, it might’ve just been a dust cloud. They’d have to wait another 164 years to see it again in order to confirm that it was indeed an exoplanet.
In addition to the other answers here already, consider that most of the exoplanets we’ve found are also huge. Again, it is because it is easier to detect them, rather than because all exoplanets are that big.
It’s selection bias. It’s not that exoplanets are all like, it’s that those are the planets that are easiest for us to detect so that’s mostly what we see. One of the main ways we detect exoplanets is by looking at the “wobble” of the star as the planet’s gravity tugs on it. A planet that’s closer to it’s star (and thus with a shorter orbit) is going to tug on its star more, which makes it more likely that we’ll be able to detect it.
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