The internet is abuzz with the new evidence of the possible ninth planet in our Solar System. With those nine objects, the asteroid belt, the Kuiper belt, and everything else in our Solar System - the number of objects which orbit our star is pretty massive. Is this odd for a star its size? It's known that our Sun is a comparatively small star, so does having so many planets make it an oddity? Or do larger stars tend to have dozens or planets, even hundreds?
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$\begingroup$ This is a beautiful question. It's a shame that it's been viewed 36 times and got only 4 up-votes!! $\endgroup$– DumbledoreJan 22, 2016 at 0:53
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7$\begingroup$ The Sun is a comparatively large star. $\endgroup$– ProfRobJan 22, 2016 at 13:37
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1$\begingroup$ Agreeing with the comment from @RobJeffries the Sun is not small, though it seems fashionable for some science writers to say so. It's a lot bigger than typical dwarf types and is a classic main sequence star at least. (Interesting question by the way; upvoted.) $\endgroup$– AndyJan 22, 2016 at 14:41
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4 Answers
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From an exoplanet-finding point of view, the Sun has between one and three planets.
The major exoplanet-finding techniques in current use involve watching for either periodic Doppler shifts as the planet's gravitational pull causes the star to wobble, or periodic brightness shifts as the planet transits the star. Both require that the planet is large enough and close enough to generate a measurable signal and that the orbital period is short enough to let astronomers distinguish periodic variations from one-off variations; the transit method additionally requires that the planet's orbit crosses the star from the point of view of Earth (which favors close-in orbits). Looking at the Solar System with these techniques:
- Mercury: too small
- Venus: Maybe visible
- Earth: Maybe visible
- Mars: too small
- Jupiter: Highly visible
- Saturn: Orbital period too long
- Uranus: Orbital period too long
- Neptune: Orbital period too long
- "Planet 9": Orbital period too long
If you look at this graph of exoplanet discoveries, Jupiter is solidly in the cluster of blue Doppler discoveries, Saturn is just past the "we've been watching for one full orbit" right-hand edge of that cluster, Earth and Venus are somewhat below the sloped minimum-period-mass line, and everything else is nowhere near the detection range.
The reason the Sun has far more known planets than any other star is simply because we've got a better look at it.
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2$\begingroup$ See astronomy.stackexchange.com/questions/8605/… $\endgroup$– ProfRobJan 22, 2016 at 13:35
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$\begingroup$ @Mark Can you please provide a link to that the official system of classification which states that planets in our local solar system are to be treated so differently to exo-planets? $\endgroup$ Sep 12, 2022 at 19:34
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$\begingroup$ @Mark, you listed nine objects, then ruled most of them out as planets ''From an exoplanet-finding point of view,' saying the Sun has between one and three planets.' Particularly since at poor little Pluto's trial and condemnation, what constitutes a planet came under intense scrutiny yet the result was clearly that nine planets were reduced to eight, not 'between one and three…' what do you mean? $\endgroup$ Sep 13, 2022 at 11:49
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$\begingroup$ @RobbieGoodwin, there's no "official system of classification" I'm using, I'm just analyzing our solar system using current exoplanet-finding methods. Mercury and Mars are much too small to see using any current technique. Most techniques require watching a star for one full orbit of the planet; of the gas giants, only Jupiter has completed one full orbit since we started watching (Saturn will complete its first orbit two years from now). This leaves just Venus and Earth, both of which are about one order of magnitude smaller than anything we've detected, which is why I called them "maybe". $\endgroup$– MarkSep 13, 2022 at 22:05
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We can't detect whether there are or are not objects of similar "small" sizes orbiting other stars. We simply lack that level of ability. From a scientific point of view I'd simply say "don't know". From a practical point of view I would expect other stars to have systems with a huge variety of things in orbit - there's no reason not to expect this, but it's bad science to assume it.
answered Jan 21, 2016 at 20:45
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$\begingroup$ I agree with this. I think it's possible that many binary systems would have less gravitational stability for planets, but non-binaries, "we don't know" is the only answer. There's a pretty good chance that newly formed systems might have more on average, but some wouldn't be long-term stable. $\endgroup$– userLTKJan 21, 2016 at 22:41
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$\begingroup$ To extend this analysis I also offer up, "I would expect other stars to be teeming with life - there's no reason not to expect this, but it's bad science to assume it." $\endgroup$– OctopusJan 22, 2016 at 23:54
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Given that Kepler has detected stars with 5 or 6 planets that exist in an extremely flattened plane and much flatter than the solar system, then we can be reasonably sure that stars with more than 4 planets are reasonably common.
Given the many mechanisms for scrambling the orbital inclinations out of a perfectly flat configuration, and that seeing multiple transiting planets around a star becomes impossible even if their inclinations are a little different (e.g. only one planet could be seen to regularly transit our Sun if observed from another star), then it is likely that many if not most of the exoplanet systems we detect have more planets than are currently known.
In other words, the fact that we see some multiple transiting systems, despite them being inherently geometrically improbable, means that there must be lots of multiple planetary systems.
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Well ,I don’t think our sun does have a lot of planets ,especially for the gravitational pull that it possesses,which is far greater than the planet Saturn’s gradations influence! I do think though ,is that Saturn has a lot of moons for a relatively weak force compared to that of the sun ,yet Saturn has something like 30 or more satellites around it ,whereas our sun only has like 8 planets ,Pluto isn’t a planet .
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1$\begingroup$ The question is comparing our Sun to other stars, other solar systems. It is not comparing the Sun to other planets $\endgroup$– uhohSep 19, 2021 at 23:01