I’ve enjoyed reading a lot of SF set on bizarre worlds – ones orbiting close to dim stars, ones orbiting brown dwarfs, etc. This piqued my curiosity…
In the recent years’ discovery of extrasolar planets, the predominant run of what’s been found have been Jovians fairly close to their primaries – much closer than Jupiter is to Sol. Obviously, a Jovian in the “temperate zone” of an extrasolar “solar system” would preclude having a habitable planet in that area, owing to tidal drag and such.
But I came up with an exception to that rule. What if a terrrestrial planet were located at the Trojan Point in the orbit of such a Jovian – the L-4 or L-5 position with respect to it and the sun?
Questions:
Would this be dynamically plausible?
Assume such a planet to exist and be more-or-less Earthlike. What would having the Jovian in the same orbit but 60 degrees ahead or behind be like? What would it look like in the skies, and what sort of influence would it have on the planet?
I’d always thought (but don’t know for a fact) that such three-entity orbits were long-term unstable.
Note, though, that the reason why these are basically the only planets that we’re detecting isn’t because these are the only planets that exist, but rather because these are the only kinds of planets that we can (currently) detect. In fact, I’m under the impression that our current state of technology couldn’t detect a Jupiter-like planet if it was located as far away from its sun as our Jupiter is.
IE, there’s plenty of room in the universe for more planet sequences like ours, and the (perhaps only) reason why we haven’t located any is because we don’t currently have the technology to do so.
I’d have thought the tidal forces would be pretty serious; other than that, it would seem to be possible to at least have an earth-sized mass orbiting at those points.
So how is it we are able to detect such planets? Is it measured by any sort of effect detected on a star that such a planet would make such as gravitational pull?
Yes; because the planets don’t actually orbit the stars, they (the planets and stars) co-orbit their collective centre of mass (which happens to be quite close to the star itself, so in practical terms, the point may be somewhat moot) - so from here in earth, the star is observed to ‘wobble’ a little. The more planetary mass there is in the (extra)solar system, the further away from the star is the collective centre of mass and the greater is the wobble.
I have a feeling that planets have been detected when they partially occlude their stars too.
An extrasolar planet has been detected like this: HD 209458b. But there should be more before too long.
Anyway, I agree with Mangetout that it should be okay. However, the tidal forces should be no more significant than if it were just orbiting the star at that distance, unless I’m missing something. One thing to note is that the planet would not really tend to fall into the L4 point per se - it would orbit around it. Not unlike how the Earth orbits around the Earth-Moon barycenter. I don’t think this would cause any significant problems, though.
I suspect you’re 100% correct in what you’re saying – both the facts (which I’m pretty sure are correct) and the implications you draw from them.
However, with what we’re detecting being close Jovians, I was wondering whether the scenario of a “Trojan Point Earth” would work, and what the Jovian would look like from Earth.
You’re saying that Trojan Earth would not stay precisely in the L4 (or L5) point of the Jovian-in-terrestrial-orbit, but would produce a “rosette orbit” where it moves slightly in and out, ahead and behind from that point much as the Moon orbits the Sun in a path mediated by its apparent motion around the Earth – a sort of “edge of a doily” effect, right?
Well, all I know is that the L4 and L5 points are potential minima in the rotating frame. Based on that alone, I would say yes, however, there’s no need for the mini-orbit to be in the same plane as the main orbit, so you’d have a few possibilites. It could be like the edge of a doily, or it could be like the edge of a coffee filter, or it could be a helix.
sevenwood is basically right about why we’re detecting “hot Jupiters” - it’s a selection effect - but their existence was kind of surprising. According to this chart there has been one planet detected as far out as Jupiter (5.2 AU), but it was four times Jupiter’s mass. NASA’s Terrestrial Planet Finder will use specialized imaging techniques to locate Earth-like planets.