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How many planets per star?
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| AeroWolf | Date: Sunday, 26.07.2015, 09:21 | Message # 1 |
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Observer
Group: Users
United Kingdom
Messages: 13
Status: Offline
| Is it possible for a star to have 100 or 1000+ planets in a stable orbit?
Also I got told that gravity stretches out infinitely. So for example if I put a star in an empty universe and put an object lets say 100 lights years away, would the object still orbit the star but very slowly?
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| HarbingerDawn | Date: Sunday, 26.07.2015, 15:05 | Message # 2 |
 Cosmic Curator
Group: Administrators
United States
Messages: 8717
Status: Offline
| There isn't a strict limit on how many planets a star can have, it all depends on the mass of the planets, the mass of the star, and the spacing between the planets' orbits, as well as the local stellar density.
Yes, hypothetically, a planet could orbit 100 ly from a star in the middle of an intergalactic void, but keep in mind that it would take an extremely small gravitational change to pull the planet out of orbit.
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| Watsisname | Date: Sunday, 26.07.2015, 21:41 | Message # 3 |
 Galaxy Architect
Group: Global Moderators
United States
Messages: 2613
Status: Offline
| Yep, the force of gravity decreases with the square of the distance (so if you double the distance, the force drops by a factor of 4, and if you triple the distance, it decreases by a factor of 9), but it never decreases to zero. So you can have arbitrarily large orbits, if there are no other significant competing gravity sources. The orbits just become very long and slow.
However, at 100LY, I think we're approaching the point where the expansion of the universe might actually qualify as "sufficiently strong competing gravity source", and possibly separate the planet from the star. 
For a planet orbiting a one solar mass star at 100LY, the orbital speed is only about 12m/s (for an orbital period of about 16 billion years, longer than the universe has existed!). The universe's expansion rate (the Hubble Constant) is about 68km/s/Mpc, or about 2m/s per 100LY. So the magnitude of the expansion rate is a fairly substantial fraction of the planet's orbital speed, and it's probably enough to break the orbit.
Of course, galaxies aren't limited to ~100LY in size because they have much stronger gravitational fields, being the sum of a very large number of stars.
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