For example:
55 Cancri b: 0.7 days
Kepler-1649c: 19.5 days
Gliese 581g: 32 days
Compare it to Earth, which is 365 days and Mercury which is 88 days. What are the factors that cause these exoplanets revolve much faster?
In: 5
Simply distance from their parent star. The closer they are then the faster that they will orbit. This is why the Earth orbits the Sun at 30kps but Neptune orbits at only about 5kps.
If they were orbiting slower then they would fall into a closer orbit if not into the Sun itself.
It’s unlikely that most of the exoplanets that we’ve discovered so close to their parent formed there since most of them are most likely gas giants which form further out. So what has most likely happened is that some sort of event has caused them to migrate closer to their parent star and therefore a faster orbit.
Now, there is a bit of some sort of selection bias here because we’re much more likely to discover an exoplanet that orbits closer to its parent star because their closer orbits makes them easier to detect.
Some of it is an observation bias. We are much better at finding extrasolar planets that orbit very fast than very slow. This is because we don’t directly see the planet. We see the blocking of light from the star as the planet moves between us and the star. That is easier when the planet is big and close to the star and does it often.
The orbital time depends on the mass of the larger object and the distance to it.
If we just look at it for objects a lot smaller from what they orbit
The time of a orbit is t = 2 x pi x sqrt( a^(3) / (G x M))
a is the semi-major axis, which is the longest distance to the large object in the orbit.
G is the universal gravitational constant of 6.67430 ×10^(−11) N m^(2) kg ^(-2)
M is the mass of the larger object
Let’s plug in the number for the earth. M is the mass of the sun 1.988 x 10 ^(30) kg. a is earth semimajor axis of 149.60×10^(9) meter
The result is T=2 x pi x sqrt ( (149.60×10^(9) )^(3) /(6.67430 ×10^(−11) x 1.988 x 10 ^(30) )) =3.15621 × 10^(7) seconds
3.15621 × 10^(7) /60/60/24 = 365.3 days. The number I have used is not exact, the solar mas has only 4 digits so we are off but 0.046 days=66 minutes from 365.256363004 days
Plugin the number of mercury with a= 57,909,050 km and you get 87.98 days.
The start of 55 Cancri b is 55 Cancri A with a mass of 0.905x out sun. The semimajor axis is 0.115x it for earth
If plugin the number I get 1.29386×10^(6) seconds = 14.97 days. That is off by 0.5 days from the number Wikipedia list for [55_Cancri_b](https://en.wikipedia.org/wiki/55_Cancri_b) not the “55 Cancri b: 0.7 days” listed in the post
[55_Cancri_e](https://en.wikipedia.org/wiki/55_Cancri_e) do have an orbit of 0.7365474 day but it is only at 0.01544x earth distance from the sun and with the formula above I get 0.74 days
It’s something of a selection bias in how scientists detect extrasolar planets. When a planet passes in front of it’s star, it causes that star to dim ever so slightly. That dimming is what we’re looking for. It takes 3 of these dimming events in order to confirm a detection, so if you were an alien trying to detect Earth, it would take you at least 3 years. It would take you 90 years to confirm the detection of Saturn.
So obviously this method has a bias in favour of planets that orbit weirdly close to their parent star, because you can rack up 3 dimming events much more quickly. So it’s probably not that our solar system is weird in that it doesn’t have any planets super close to the sun, it’s just that our detection methods tend to find the ones that do.
Something important to keep in mind when thinking about exoplanets is that both the planets in our solar system and the exoplanets that people are detecting are probably very atypical in different ways.
Some exoplanets are much easier to detect than others using current technology, for example large planets orbiting small stars are generally easier than small planets orbiting large stars. So the known exoplanets are almost certainly not representative of planets in general.
This is more speculative, but it’s likely that some kinds of planets and stars are far more hospitable to life than others. There’s a lot of debate about what the important factors are exactly. But it’s reasonable to suspect that our own solar system might be “weird” in some way that makes it especially suited to the development of life. In particular, it has been suggested that planets that orbit very close to their star, which orbit very quickly, would probably be tidally locked – that is, one side always faces the star and is always very hot, while the other side always faces away and is always very cold. Some people think that may make it difficult for life to develop.
This is mostly about distance from the star. The closer something is to the star, the faster it orbits it (and the less time it takes).
Our methods of detecting planets have a much easier time finding planets that are close to their stars, so we tend to spot a lot of fast-orbiting planets.
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