How can they detect extrasolar planets?

Question

I've heard of the blinking of a star when a 'planet' supposedly passes in front of it.

Or the wobble of a star, when a planets gravity pulls on it's host star.

I don't understand how that is plausible. If you were on another solar system and looking at ours, how would you separate this 'wobble' from Earth, Venus, Jupiter, Mars, and also calculate the distance each planet is from the sun if you were on another solar system?
And how could you say that Jupiter is as far away from the sun as it is, as opposed to being a much smaller planet, much closer to the sun than it is. Simply by it's effect on the sun?
Help me out here. I think it sounds like scientists getting overzealous and jumping to conclusions to find that 'holy grail' of Earth like planets.

Answer 1

If you were watching our sun from another planetary system, here is what you would see:

http://innumerableworlds.files.wordpress.com/2009/04/sunwobble.jpg?w=460&h=431



As for how you would distinguish the individual planets...that is the biggest challenge of data analysis that there may be in all of astronomy (in my opinion). BUT, indeed it can be done.



It is a trivially easy task to construct that diagram that I showed you. A college freshman could do it, or should be able to do it, should I give said person all timing data of solar planet positions. All it is, is simple Kepler's laws, and basic usage of the definition of center of mass.





However, given nothing more than data that would produce a diagram like that and trying to figure out the masses, apastrons, periastrons, and timing of a distant planetary system's planets...that is very difficult.



Fortunately, when you incorporate computer science with astronomy, then you can get some aid. Essentially what one would need to do, is to write a program that would simulate such a diagram, search for the major contributors, solve their masses and orbits via simple Kepler's laws...AND THEN, once the big contributors are determined, then the program can continue to buffer out the anomalies.



A lot of simulation, a lot of algebra, a lot of brute-force search algorithm writing.



It could take a good fraction of one's entire career to write such a program. And in fact it probably did, when the idea of how to search for planets was proposed.



BUT, now that the software is written, the discoveries can continue at ever rapidly increasing rates. Now that we know what we are looking for, it is easy to move onward.







You can see that indeed it is easier to find massive planets, and that Jupiter is a major contributor to sun wobble. Sol is the parent star we analyze, and we call Sol's system, the solar system.





If you sequence the way that an extrasolar astronomer would keep track of our planets, according to our nomenclature, you would have:

Sol-b would be Jupiter, probably followed by Saturn being Sol-c.



Perhaps Sol-d would be Earth, and then followed by perhaps Venus as Sol-e.



Mars and Mercury would likely just not be massive enough to get easily discovered.



Uranus and Neptune would probably take at least 50 years to even notice enough anomalies to know about them, discovering them this way. This is precisely why other planetary systems don't seem to have a lot of Neptune-like planets orbiting in their outer zone. We simply aren't patient enough to wait to know that they exist. We want to point out the ones we can immediately observe.

Answer 2

Well, when you get a planet like Jupiter - fully 9% of the mass of the sun - it makes for a detectable 'wobble' - especially if the wobbling occurs very rapidly... remember, the first Extrasolar planets discovered were orbiting closer to their stars than Mercury is to our sun - with a "year" being only 3 to 10 days long.



Jupiter is *much* further our from our sun, and takes years to make a single orbit - the "wobble" our sun does is less pronounced, because Jupiter is so far away, and much, much slower - taking 12 years to complete *one*.



Now, judging by how fast the star wobbles, you know the exo-planet's orbital period, and because you can guess a star's mass by it's color, you can tell how massive that planet must be.



Just wait... in a couple of years, we'll not only be able to tell you if there are Earth-size planets around a star, but also if there's life on them - by how the light from their star is filtered through their atmospheres....

Answer 3

First of all, we're way too early in the detection phase to even begin to determine whether or not, and if so to what degree, the facts about exoplanets fit ANY estimates we may have made. Second, that is not a calculation, but a guess - and not an educated one. It sounds to me more like something invented by a bad sci-fi author. An educated guess would use the potato chip rule, and the average size of stars and their habitable zones to form a model. Stating that stars outnumber planets already violates that. Furthermore, it would be impossible at this point to come up with an educated guess as to how many habitable planets actually do host life, let alone intelligent life or anything further along down that list.

Answer 4

In the "wobble" technique, the first step is to get an estimate of the mass of the star. They can actually do that by looking at the star's light, because it turns out there's a relationship between the star's luminosity and color, and its mass (see "mass-luminosity relationship" and the "Hertzsprung-Russell diagram")



Once you know the star's mass, you can actually calculate the planet's distance from the sun by timing the duration of the wobble. That's because the duration of the wobble is the same as the duration of the planet's orbit, and there is a mathematical relationship between a star's mass, the duration of a planet's orbit, and the planet's distance from the star (this principle was first discovered by Kepler and later refined by Newton).



The final step is to look at the star's "doppler shift". As the star wobbles, it alternately swings back and forth, first farther then closer to the earth. When things move back and forth like this, it slightly affects the wavelength of the light they emit. By carefully analyzing these shifts in wavelength, scientists can tell how fast (miles per hour) the star is swinging back and forth. And it turns out that this speed is related to the planet's mass, the star's mass, and the distance between the two. So at that point they can plug it into a formula and determine the planet's mass.

Answer 5

They can find the largest extrasolar planets (mostly gas giants) by the "winking" effect when they pass in front of their star's light. Terrestrial planets are much harder to find because of how small they are.



Planets are extremely faint light sources compared to their parent stars. At visible wavelengths, they usually have less than a millionth of their parent star's brightness. It is extremely difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out.

Direct image of exoplanets around the star HR8799 using a vector vortex coronagraph on a 1.5m portion of the Hale telescope



For the above reasons, telescopes have directly imaged no more than about ten exoplanets. This has only been possible for planets that are especially large (usually much larger than Jupiter) and widely separated from their parent star. Most of the directly imaged planets have also been very hot, so that they emit intense infrared radiation; the images have then been made at infrared rather than visible wavelengths, in order to reduce the problem of glare from the parent star.



At the moment, however, the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful:



* Radial velocity or Doppler method



As a planet orbits a star, the star also moves in its own small orbit around the system's center of mass. Variations in the star's radial velocity — that is, the speed with which it moves towards or away from Earth — can be detected from displacements in the star's spectral lines due to the Doppler effect. Extremely small radial-velocity variations can be observed, down to roughly 1 m/s. This has been by far the most productive method of discovering exoplanets. It has the advantage of being applicable to stars with a wide range of characteristics.



* Transit method



If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. This has been the second most productive method of detection, though it suffers from a substantial rate of false positives and confirmation from another method is usually considered necessary.



* Transit Timing Variation (TTV)



TTV is a variation on the transit method where the variations in transit of one planet can be used to detect another. The first planetary candidate found this way was exoplanet WASP-3c, using WASP-3b in the WASP-3 system by Rozhen Observatory, Jena Observatory, and Toruń Centre for Astronomy.[30] The new method can potentially detect Earth sized planets or exomoons.[30]



* Gravitational microlensing



Microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. Planets orbiting the lensing star can cause detectable anomalies in the magnification as it varies over time. This method has resulted in only a few planetary detections, but it has the advantage of being especially sensitive to planets at large separations from their parent stars.



* Astrometry



Astrometry consists of precisely measuring a star's position in the sky and observing the changes in that position over time. The motion of a star due to the gravitational influence of a planet may be observable. Because that motion is so small, however, this method has not yet been very productive at detecting exoplanets.



* Pulsar timing



A pulsar (the small, ultradense remnant of a star that has exploded as a supernova) emits radio waves extremely regularly as it rotates. If planets orbit the pulsar, they will cause slight anomalies in the timing of its observed radio pulses. Four planets have been detected in this way, around two different pulsars. The first confirmed discovery of an extrasolar planet was made using this method.



* Timing of eclipsing binaries



If a planet has a large orbit that carries it around both members of an eclipsing double star system, then the planet can be detected through small variations in the timing of the stars' eclipses of each other. As of December 2009, two planets have been found by this method.



* Circumstellar disks



Disks of space dust surround many stars, and this dust can be detected because it absorbs ordinary starlight and re-emits it as infrared radiation. Features in the disks may suggest the presence of planets.