Methods used to find exoplanets

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5hot6un

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The method of observing the wobble of a star to detect an orbiting planet leaves me scratching my head.

Is it assumed that there is only one planet causing the wobble? Wouldn't all the planets around a star create the wobble? How are multiple planets accounted for?

I am also surprised to hear about exoplanets that orbit their star in a few days. How could this even be possible? Why are the planets in our solar system orbiting so slow in comparison?
 
K

kg

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The wobbles of a star are detected by looking at lines in the stars spectrum. As a planet pulls the star away from us when its orbit takes it to the opposite side of the star from us the spectral lines move toward the red end of the spectrum (red shifted). When the planet pulls the star towards us these lines move toward the blue end of the spectrum (blue shifted). The reason why so many of the planets found so far are very large and orbit very close to the their host star is that these planets are much easier to detect with this method. Smaller planets make the star wobble less and give a weaker signal. Planets orbiting further from the star take observations over longer periods of time to detect. What also needs to be taken into account with this method is the rotation of the earth, the gravitational influence of the moon and planets and the earths orbit around the sun! Over time as the number of observations increase the wobbles from multiple planets around a single star get teased out of the data.
The planets found so far do seem very strange but I haven't heard of any of them defying the laws of physics.
 
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ramparts

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5hot6un":2ohrk9vk said:
The method of observing the wobble of a star to detect an orbiting planet leaves me scratching my head.

Is it assumed that there is only one planet causing the wobble? Wouldn't all the planets around a star create the wobble? How are multiple planets accounted for?
Yes, that's assumed to an extent. kg explained how we see the wobble. Now, we see this wobble happening periodically - so, if we were looking at the Sun from afar, we might see the Earth causing it to wobble back and forth every 365 days. No other planet will make it wobble with that time period, so we can be pretty sure that there's one planet there. However, multiple planet systems exist, of course, and have been detected with this method. We just see different periodicities in the wobble data :)

I am also surprised to hear about exoplanets that orbit their star in a few days. How could this even be possible? Why are the planets in our solar system orbiting so slow in comparison?
Well, because we have a different type of solar system. It's no accident that all the solar systems we've found are so different from ours - the wobble method will most easily find heavy planets really close to their stars. All of our heavy planets are a lot further out, and the stuff that is close in is a) not as close in as these "hot Jupiters" we find with the wobble and b) way too light to cause a major wobble. Our technology is getting better, and perhaps one day the wobble method will allow us to find solar systems similar to ours. But if we put our telescopes on even a nearby star and looked at the Sun, we'd maybe see Jupiter, that's about it.
 
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5hot6un

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I recall hearing about Carl Segan postulating the number of possible planets in our Milky Way. The point he was making was that even the most conservative estimates of stars with planets would still predict a large number of planets. And of those possible planets, a conservative estimate of ones that could support life would still calculate to a very high number. And a conservative estimate of the life supporting planets that had intelligent life would equate to a significant number. IIRC this was the premise for SETI.

So my question is, now that we are discovering exo planets, are we finding Carl's estimates of stars with planets to have been overly conservative? Personally, I would expect a star without planets to be a rare thing. Especially in the outer regions of a galaxy.
 
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MeteorWayne

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There was another discussion just afew days ago about this; I may merge this into the other one.

Each planet causes its own wobble, using mathematics (Fourier Transform) you can separate the different planetary periods. The reason only short period planets have been found so far is that we've only been looking for a decade or so. In order to reliably separate the different orbital periods of a multiple planet system, you need several orbits. Therefore we have only detected really short periods from large planets (easy to find) and planets of reasonable mass with periods of a few years at most. For example, our search has not lasted long enough to find even Jupiter in our solar system, much less Uranus or Neptune.

Wayne
 
S

SpaceTas

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Best current estimate of the frequency of planets:
About 35% of stars have medium to Jupiter mass planets. This holds for hot Jupiters and cold planets.

There are indications there are
much fewer very massive planets.
more planets at larger distances ie beyond the "snow line" (= where water can condense, In our solar system between asteroid belt and Jupiter)
only 1/6 of systems are like our solar system (multi-giant planet)

Info taken from a paper from the micro-Fun group summarizing recent results.
Gould et al 2010 "Frequency of Solar-Like Systems and Planet Mass-Ratio Distribution Function Beyond the Snow Line from High-Magnification Microlensing Events". Results were presented at the AAS. Here is a link to the OSU press release:
http://researchnews.osu.edu/archive/noplanet.htm
 
S

SpaceTas

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Planet detection methods:

Radial Velocity (Doppler, wobble)
best at finding high mass close in planets. Will be able to find lower and lower mass planets in longer orbits.
Yields period, lower limit to mass. Limited by activity of star, and stars cooler than A-type.
Has found most planets. Many multi-planet systems.

Transit method
best at finding large (in size) and so high mass planets close in to star. Kepler is space mission, that will be able find Earth mass planets in Earth length orbits. Will find smaller size and longer orbit planets. Limited by stellar activity. Measures size (not mass) and orbital period of planet.
Has found lots of planets. Possible to detect exo-moons

Microlensing
Best at finding medium to large mass planets beyond the snow line. Found 17 planets. Limited by seeing quality, and telescope time. With space mission would be able to find sub Earth mass planets.

Direct Imaging
best at finding large, high mass planets far from their host star. Also good at finding young therefore hot planets far from star. Currently limited by technology. Possible major limit is brightness of exo zodiacal light. Several planets found, including first direct spectrum. Team up with spectroscopy and spectropolarimetry to characterize surfaces/atmospheres of planets.

Astrometry
Best at high mass planets far from planet. No firm detections (VB 10 withdrawn). Will require space mission. Limited by technology, and stellar activity.

Timing
Best at finding short period planets. Needs stable clock (pulsar, pulsating white dwarf, short period pulsator) for host star. Has found several planets including lowest mass planets (moon mass).

Radio
Possible to detect the magnetospheres of planets (most likely giants) with low frequency observations. Maybe find a few with LOFAR. Of course there is always a possible detection of a intelligent signal (also in optical).

Serendipity
Finding evidence of super high tech civilization. Dyson spheres etc
 
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5hot6un

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SpaceTas":1ndxjo0u said:
Planet detection methods:

Radial Velocity (Doppler, wobble)
best at finding high mass close in planets. Will be able to find lower and lower mass planets in longer orbits.
Yields period, lower limit to mass. Limited by activity of star, and stars cooler than A-type.
Has found most planets. Many multi-planet systems.

Transit method
best at finding large (in size) and so high mass planets close in to star. Kepler is space mission, that will be able find Earth mass planets in Earth length orbits. Will find smaller size and longer orbit planets. Limited by stellar activity. Measures size (not mass) and orbital period of planet.
Has found lots of planets. Possible to detect exo-moons

Microlensing
Best at finding medium to large mass planets beyond the snow line. Found 17 planets. Limited by seeing quality, and telescope time. With space mission would be able to find sub Earth mass planets.

Direct Imaging
best at finding large, high mass planets far from their host star. Also good at finding young therefore hot planets far from star. Currently limited by technology. Possible major limit is brightness of exo zodiacal light. Several planets found, including first direct spectrum. Team up with spectroscopy and spectropolarimetry to characterize surfaces/atmospheres of planets.

Astrometry
Best at high mass planets far from planet. No firm detections (VB 10 withdrawn). Will require space mission. Limited by technology, and stellar activity.

Timing
Best at finding short period planets. Needs stable clock (pulsar, pulsating white dwarf, short period pulsator) for host star. Has found several planets including lowest mass planets (moon mass).

Radio
Possible to detect the magnetospheres of planets (most likely giants) with low frequency observations. Maybe find a few with LOFAR. Of course there is always a possible detection of a intelligent signal (also in optical).

Serendipity
Finding evidence of super high tech civilization. Dyson spheres etc
Outstanding! Thank you very much!
 
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