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Ask an Astrophysicist: Exoplanets

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Exoplanets

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Library of Past Questions and Answers

    Exoplanets

Can you tell me more about those new planets that were found? (Submitted October 24, 1996)

The Question

Can you tell me more about those two new planets that were discovered?

The Answer

To answer any question you have about planets which have recently been discovered orbiting other stars, there are a number of places to look.

You might want to look at the article by David Black in the August 1996 issue of Sky and Telescope. We think David does a very nice job of describing what we know now and what might be learned in the next few years.

There are also a number of good web sites. Jean Schneider, of the Observatoire de Paris, runs the Extrasolar Planets Encyclopedia. There are a number of pages run by extrasolar planet research groups themselves. Marcy and Butler work out of San Francisco State University, and Alex Wolszczan, the first to discover an extrasolar planet, works out of Penn State.

Question ID: 961024

How common are planetary systems around other stars? (Submitted September 22, 1997)

The Question

How common are planetary systems around other stars?

The Answer

The evidence is mounting that planets are quite common around other stars. Because the mass of any planet around it's parent star is much less than the star itself, it is difficult for us to observe the effects of the planets from Earth, and it really isn't possible right now to make an intelligent estimate of the percentage of other stars that have planets. However, a number of different observational studies have results that when combined imply that planets are the rule and not the exception. For example, theorists believe that planets form from a disk of material circling a star when it is young. Observations of young stars with the Hubble Space Telescope and other instruments have directly imaged such circumstellar material. Very young stars are also found to show evidence of jets coming out of the poles---jets are a very strong indicator of a disk structure.

Researchers who study the precise timing of pulsars have found that some pulsars show a wobble in the period of the pulsar. That is, instead of the pulsar always having the same period, sometimes it is slightly faster, and other times slightly slower. This is strong evidence for orbital motion of the pulsar about the center of mass of a system. From careful analysis of the pulsars' period changes, orbits have been deduced that suggest planets circle the pulsars.

Within the last two years or so, a number of research collaborations, including ones in the US, have found compelling evidence for Jupiter-sized planets around relatively nearby Sun-like stars. These systems are mush more similar to the Sun and our planets than are the systems around pulsars. These teams use very precise measurements of the stars' radial velocity, that is, the speed at which it is moving toward or away from us. If the star has planets a very small wobble will be seen in the radial velocity. They show that many stars may host Jupiter-like planets, some in very close, or very eccentric orbits. The hunt is on for more planets around Sun-like stars, now that the technique has proven fruitful!

So, while I can not give you an exact numerical answer to your question, I hope I have given you an idea of why astronomers have recently begun to suspect planets around many stars.

For more information, check out

http://www.obspm.fr/department/darc/planets/sites.html

There is a section on extrasolar planets in WebStars with links to sites elsewhere on the subject. This, and many other topics, are discussed in WebStars at

http://heasarc.gsfc.nasa.gov/docs/www_info/webstars.html

In particular, there is information on pulsar planets at

http://www.astro.psu.edu/users/alex/pulsar_planets.htm

Cheers,

Padi Boyd
for the Ask an Astrophysicist

Question ID: 970922f

Can you tell me about the new solar system that has been found? (Submitted April 19, 1999)

The Question

I am in the 7th grade, and have a great interest in astronomy. A couple of days ago, I read an article in the newspaper about a new solar system that has been found, with multiple planets orbiting a star. Could you tell me where to find more information on this?

The Answer

The star in question is Upsilon Andromedae. The discovery of 2nd and 3rd planets is very new, in fact this hasn't been published yet in a proper form. (The discovery of the first planet was announced in 1997).

The group who announced this discovery has a web site on it: http://www.physics.sfsu.edu/~gmarcy/planetsearch/upsand/upsand.html

A good site for extrasolar planets in general is:

http://exoplanets.org/

Hope this helps,

Koji Mukai
for "Ask an Astrophysicist"

Question ID: new_planets

How are solar systems created? (Submitted February 15, 1998)

The Question

How is a solar system such as ours created?

The Answer

A solar system is created when a rotating cloud of gas and dust in space start to coalesce - they are pulled together and towards the center of the gas/dust cloud by their gravitational attraction to each other. As they condense, the particles collide faster and more often, which causes the gas and dust to heat up. The gas and dust at the center collapses to form the central star of the solar system; the heat generated by the colliding particles starts nuclear fusion in its core. If there was enough angular momentum in the system at the very beginning, then not all of the dust and gas will go into the central star - the rest will remain in a flattened disk around the star. The planets form from this disk of rotating material as it clumps together because of gravity.

There are a number of places on the web where you can go to for a more detailed description of the process, and even some numerical simulations of this.

J. Allie Hajian
for Ask an Astrophysicist

Question ID: 980215a

Can you tell me the differences between exoplanets and brown dwarfs? (Submitted October 23, 1997)

The Question

Can you tell me the differences between exoplanets and brown dwarfs please. Also I could not find any book regarding this. Is it because brown dwarfs and exoplanets are new to us?

The Answer

This is somewhat outside our field of expertise (which is X-ray, gamma-ray and cosmic ray astrophysics), but I've managed to find a definition in the 'EXOPLANETS' pages at:

http://exoplanets.org/

The difference is in how they formed: brown dwarfs formed through the collapse of a molecular cloud, planets formed around a protostar through accretion of planetesimals and gas.

I think the subject of (theoretical studies of, and searches for) brown dwarfs and exoplanets have been with us for a long time. However, until recently, we had had very little data --- it is only within the last few years that many important discoveries have been made. So I think you are basically correct as to why you cannot find books on these subjects.

Best wishes,

Koji Mukai
for Ask an Astrophysicist

Question ID: 971023e

How can you separate multiple exoplanets in radial velocity measurements? (Submitted July, 21, 2010)

The Question

I've read something about how astronomers discover exoplanets. I understand the basic concept - the star is bright and the planets are dim so you cannot see them directly. But you know that if there are planets, the star will wobble a bit because the planets have a gravitational effect on it. OK, this is fairly clear. However, you cannot know how many planets orbit the star. Would the wobble not be highly erratic if there were, say, 10 planets, possibly with irregular orbits, pulling the star to and fro? Do you have tools to separate the impacts of individual planets? Or does the current technology only permit to find a planet if it is the only planet circling its star?

The Answer

This is an excellent question! The answer is that yes, the motion of the star becomes quite complicated when there are multiple planets to consider However, we have very good mathematical tools for separating out the effects of each planet.

Consider the simple case of a planet in a perfectly circular orbit. The motion it imparts to the star will then have the shape of a sine wave, v = A * sin( omega * t ), where A is the amplitude of the effect and omega is the rate at which the planet orbits the star. This is illustrated in the example on the page:
http://en.wikipedia.org/wiki/Doppler_spectroscopy

Now, if there is another planet orbiting the star, it will be at a different distance from the star, and hence omega will be different. So now you have the sum of two sine waves at different frequencies, and so on for further planets in the system. Each additional planet is one additional frequency that will show up in the signal (all at different amplitudes as well, but that doesn't help us separate them). Fortunately, it is easy to separate out multiple sine waves at various frequencies. The mathematical tool used for this is the Fourier Transform, which gives you all the frequencies and amplitudes in one operation. It works very much like your ear does when you hear multiple tones at once.

There are two complications that make reality a little more work, however First is that the Fourier Transform is not actually the best tool for most accurately determining A and omega, so somewhat more sophisticated tools are actually used. These basically involve calculating the shape of the expected sine wave as a function of frequency, and then trying various values of amplitude and frequency until you find the best match to the actual signal. You may recall Newton's Method of finding a solution to an equation; that is basically what is done here, but with two variables.

The second complication is that the planets may not have circular orbits. In fact, they are always at least somewhat eccentric (i.e. elliptical). This means that each planet's signature is not quite a sine wave, and thus the Fourier Transform doesn't work perfectly. For orbits that are only slightly eccentric, the Fourier Transform still works pretty well, but for very eccentric orbits it becomes less sensitive. In those cases it's really required to use the search algorithm described above, but with eccentricity included as a third value that is varied while searching for the best fit to the data.

There are also some other methods of finding exoplanets besides mapping out the wobble of the star by doppler spectroscopy. Until now that has been the most successful method, but with the recent launch of the Kepler mission, the transit method is being used. There is a nice article about all the methods of finding exoplanets at:
http://en.wikipedia.org/wiki/Methods_of_detecting_extrasolar_planets

We hope that helps.

-Kevin and Ira,
for "Ask an Astrophysicist"

Question ID: 100721a


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