Exoplanet Wobble: Moons accounted for?

[lohtse101]

In the search for exoplanets, the size of what we've found, especially the "hot gas giants" that are close to their primaries, makes me wonder if moons have been taken into consideration for the wobble method (if it's even possible). It seems to me that a planet with 10 Earth masses, for instance, would have the same general effect (at our level of ability to resolve the detail) as an 8 Earth-mass planet with a 2 Earth-mass moon when looking at the wobble of a star.<br /><br />The presence of significant lunar masses may also impact the findings of a "transit" method in that the net drop of visual energy would be the same for a planet and moon as it would be for just a single large planet with the same diameter as the a planet and moon put together. However, in this case, I would assume that there should be some sort of variance thrown into the equation by the fact that the moon(s) are not only transiting the primary with the planet but also transiting the planet itself (e.g. a dimming and brightening cycle as the planet crosses the face of the star). However, differentiating that in the short amount of time we have during a transit would be pretty difficult, I'd imagine.<br /><br />Of course, as I think of this, I'd have to wonder if moons would truly be an impact when looking at something that is evidently multiple earth masses. Yet, I still find it interesting to consider that Gliese 581 C, at 5 times more massive than Earth, could possibly be, say, four times more massive than Earth but with a moon that is equal to 1 Earth mass...

 

[qso1]

Welcome to SDC, good first post.<br /><br />I'm no expert on planetary detection methods but I'm not sure the techniques are refined enough to detect the motion of satellites around exoplanets.<br /><br />Even if they are, I suspect that this type of motion would be taken into account. The larger motion offset being the planets tug on the parent star. The satellite would tug on the planet so to speak as it orbits the star but the orbital size of planet around star is typically much larger than satellite around planet.<br /><br />In addition, so far there have been no announcements from exoplanet hunters of a planetary companion that I'm aware of. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[vogon13]

Computer models of transit light curves simulating various ring and satellite configurations have been generated and compared to actual transit light curves.<br /><br />So far, nothing 'fun' has turned up.<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[heyscottie]

I'd have to say you are probably right. Of course, the wobble method used can only estimate a minimum mass for the planet detected -- the values could be much higher if the orbit is close to perpindicular to Earth's viewpoint.<br /><br />I would think that the wobble method would be incapable of differentiating between a single planet, a binary planet, a planet with several moons, etc. It would only be able to detect the presence of some amount of mass in a defined orbit.<br /><br />I am also curious how the estimate that the planet is "about 50 percent larger" than Earth was made, too. They must be assuming a density for a planet of that mass range.

 

[lohtse101]

Thanks for the great replies. It's extremely interesting to me that, Vogon13, there has been no evidence of the rings and satellites with the many planets we've found. That seems a statistical oddity to me, in light of all the orbital accoutrements the planets of our system have. Could there be some natural law that once a planet gets big enough, any companion it had would have had to move fast enough to not be absorbed that it achieved escape velocity?<br /><br />Also, would tides be enough to make a star appear to wobble more than it really is? Is there such a thing as a tide in a star mass due to an orbiting planet? I could imagine some effect in transit observations where it would make the planet seem bigger than it really is.<br />

 

[vogon13]

Saturn type rings won't be found near any star hugging planet as they are primarily icy in composition.<br /><br />Silicate dust rings would be subject to Poynting Robertson effects and their orbital lifetimes would be short. Additionally, close proximity to the star would generate largish perturbations in the ring particle's orbits, increasing their orbital eccentricities, grinding the particles smaller, and increasing the effectiveness of the Poynting Robertson effect.<br /><br />Frankly, finding a set of rings on one of these extrasolar star hugging planets would be much more mysterious than not finding them is.<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[MeteorWayne]

That's true for 2 reason.<br /><br />1. Rings have a very short lifetime in most cases, relative to the age of a stellar system.<br /><br />2. We can just barely detect planets now, finding associated rings would big several magnitudes more difficult. Stellar dust rings are much larger, so the excess IR can be detected at large distances from the star. For a planet with an associated ring next to a star, seperating the ring from the planet at this point would be nearly impossible. If you've ever looked at the spectra and light curves from any of these occultation detections (which would be the only chance of discovering a ring system) the planetary information is just barely at the significance level above the noise. A ring is very wispy! <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>