The Search for Extrasolar Planets
Created | Updated Feb 23, 2005
***this is currently just the text of my (rushed) tutorial essay. I will hopefully be rewriting it and submitting it for peer review this weekend. Comments welcome :-)***
If there is life similar to our own in other parts of the galaxy or universe, it needs a planet such as our own to support it. But the problem is, how do we see them if they're there? If one looks up into the sky one can only see planets in our own solar system- ones around other stars are too small to see.
There are a variety of approaches to searching for extrasolar planets. Most of them employ indirect methods of detection, however in the future it will be possible to see extrasolar planets directly using instruments on earth and in space.
Firstly one needs to know what one is searching for. Currently there is no precise definition of what constitutes a planet, and what is a brown dwarf, or failed star. This is because there is a continuum variation with planets at one end of the scale and brown dwarves at the other, making a choice of mass where the boundary is largely arbitrary. One definition is that a planet must be larger than Pluto, but less than 12 times the mass of Jupiter, and directly orbit a star. At more than 12 times the mass of Jupiter nuclear reactions will start to take place in the centre of the body.
An example of a star found to have a companion is 51Pegasus. This was the first planet to be discovered around a normal star, discovered in 1995 by D. Queloz. However the planet itself is strange. It is half the mass of Jupiter, but orbits very close to the star. Its orbital period is only 4.2 days. This particular planet raises interesting questions, since although it is of a comparable size to Jupiter it cannot have been formed in the same way, because otherwise it wouldn't be as close to the star as it is.
The main techniques used to search for extrasolar planets are astrometric detection, radial velocity, ground-based photometry and direct observation. Most extrasolar planets are detected using the radial velocity method.
Astrometric detection is an indirect method of finding extrasolar planets. Some stars exhibit what is known as proper motion, meaning that over a period of time they appear to move in a straight line across the sky. If a planet is orbiting the star, the effect of its gravity will mean that both the star and the planet will be orbiting a common centre of mass, which will not be at the centre of the star. This will mean that over time instead of moving in a straight line a star will move in a sinusoidal motion; the star will wobble. Measurements of this wobble are then used to determine the mass of a planet, or in fact if there are multiple planets in the star's system.
A problem with this method is trying to tell apparent motion due to the atmosphere of the earth, and even instrumental defects. A possible solution is to study the stars in a cluster. It is then possible to factor out the motion that they have in common and see if there are any perturbations that could be caused by planets. Another advantage of studying clusters is that because the stars are close together it is possible to study stars at a time.
The radial velocity method works as follows: If there is a planet, the star and the planet will orbit a common centre of mass for the system. By looking at the spectrum of the star, it is possible to determine whether the star is moving towards or away from us, using the Doppler effect. The spectral lines of the star will be shifted toward the blue end of the spectrum if the star is coming toward us, and toward the red end if it is moving away. If the star periodically shifts from moving toward then away from the Earth, this will be because of an unseen companion, which is causing the system to orbit a common centre of mass. However, because the size of the planet is very small relative to the size of the star, these shifts are small. Nonetheless this is the most common method, which has been used up to now to detect extrasolar planets, and it is thought that all of the brown dwarves near to the sun have been detected mainly using this method. This method has a bias toward larger planets, of several times the mass of Jupiter.
Another indirect method is ground-based photometry- watching for occultations of stars by companion planets. When a planet passes in front of a star, the star's spectrum will change. By taking spectra of a star over a period of time it is possible to monitor to see if there are any changes that could be caused by planets. The first reported detection of a planet using this method took place earlier this year, using the ELODIE spectrograph. D. Queloz et al. detected distortions in stellar lines of the star HD209458.
Planets in the solar system don't glow; yet we can see them because they reflect light from the sun. Similarly, planets in other systems won't glow, but will reflect the light of their primary star. Looking for planets directly hasn't been used much in the past because a planet has to be very big, reflecting a lot of light, before it can be seen as different to the star that it orbits. The light of the star is many, many times brighter than the light reflected by the planet, by about a billion times. However there have been recent advances allowing direct detection of planets to be possible in the very near future. One approach is to use a coronagraph, an instrument that blocks out most of the light of the star, so that only the corona can be seen. Because there isn't the glare from the primary star present, it would then be possible to see planets in orbit. Practically, a large, ground based telescope making use of adaptive optics to overcome the problems of seeing should be able to see planets of around the size of Jupiter orbiting other stars.
Another way of refining this approach is to look at infrared rather than visual wavelengths. While a star may be a billion times brighter than its companion planet in the visual range of the electromagnetic spectrum, in the infrared the planet radiates relatively more heat. At infrared wavelengths, the planet can be only a million times less bright than the star. For example using the Keck interferometer in Hawaii hot, Jupiter like planets have already been detected close to their primary star. It is also possible to arrange the interferometer so that light from the star destructively interferes with itself, making it easier to observe planets and dust around the star.
The future for extrasolar planet searches is exciting. There are major projects planned for the beginning of the 21st century by both NASA and ESA
The Next Generation Space Telescope (NGST) is planned to replace the Hubble Space Telescope. It will have an 8-metre mirror, and will therefore be able to resolve a lot better than Hubble can. Space telescopes have the advantage that they are above perturbations caused by the Earth's atmosphere, so they do not need to be used in combination with adaptive optics, and can get a better resolution than a ground-based telescope. One of NGST's science goals is to study the formation of planetary systems.
NASA's Origins mission also plans to try to detect earth-sized planets, both indirectly with the use of the Space Interferometery Mission, and directly for stars within 15 parsecs of the sun using the Terrestrial Planet Finder (TPF). TPF will look for planets that are 'twins' of earth. TPF will use spectroscopy to determine whether or not planets have the same atmospheric composition as Earth.
The GAIA will be run by the European Space Agency. It will map the sky, determining the positions of stars, and will also be able to see larger extrasolar planets up to 500 parsecs away. Another ESA mission will be Eddington. Eddington will be a 1.2 metre space telescope, which has similar goals to the TPF. Its main mission will be to look at planets in the habitable zone around stars, which is where water can exist in liquid form. Eddington will use photometry to detect transits of a planet across a star.
BIBLIOGRAPHY
http://exoplanets.org/index.html - The Search for Extrasolar Planets, run by he Department of Astronomy at UC Berkeley
http://xxx.lpthe.jussieu.fr/abs/astro-ph/0006213 - Detection of a spectroscopic transit by the planet orbiting the star HD209458 by D. Queloz et al.
http://www.public.asu.edu/~sciref/exoplnt.htm The Search for the Extrasolar Planets: A Brief History of the Search, the Findings and the Future Implications, by George H. Bell, Arizona State University.
http://www.obs-nice.fr/baudoz/coro/coro.html Achromatic Interferential Coronagraph, Observatory of la Cote d'Azur
http://huey.jpl.nasa.gov/keck/ - The Keck Interferometer website
http://www.ngst.stsci.edu/science/ - The Next Generation Space Telescope Science Goals, from the Space Telescope Science Institute.
http://astro.estec.esa.nl/GAIA/index.html - The GAIA homepage
If there is life similar to our own in other parts of the galaxy or universe, it needs a planet such as our own to support it. But the problem is, how do we see them if they're there? If one looks up into the sky one can only see planets in our own solar system- ones around other stars are too small to see.
There are a variety of approaches to searching for extrasolar planets. Most of them employ indirect methods of detection, however in the future it will be possible to see extrasolar planets directly using instruments on earth and in space.
Firstly one needs to know what one is searching for. Currently there is no precise definition of what constitutes a planet, and what is a brown dwarf, or failed star. This is because there is a continuum variation with planets at one end of the scale and brown dwarves at the other, making a choice of mass where the boundary is largely arbitrary. One definition is that a planet must be larger than Pluto, but less than 12 times the mass of Jupiter, and directly orbit a star. At more than 12 times the mass of Jupiter nuclear reactions will start to take place in the centre of the body.
An example of a star found to have a companion is 51Pegasus. This was the first planet to be discovered around a normal star, discovered in 1995 by D. Queloz. However the planet itself is strange. It is half the mass of Jupiter, but orbits very close to the star. Its orbital period is only 4.2 days. This particular planet raises interesting questions, since although it is of a comparable size to Jupiter it cannot have been formed in the same way, because otherwise it wouldn't be as close to the star as it is.
The main techniques used to search for extrasolar planets are astrometric detection, radial velocity, ground-based photometry and direct observation. Most extrasolar planets are detected using the radial velocity method.
Astrometric detection is an indirect method of finding extrasolar planets. Some stars exhibit what is known as proper motion, meaning that over a period of time they appear to move in a straight line across the sky. If a planet is orbiting the star, the effect of its gravity will mean that both the star and the planet will be orbiting a common centre of mass, which will not be at the centre of the star. This will mean that over time instead of moving in a straight line a star will move in a sinusoidal motion; the star will wobble. Measurements of this wobble are then used to determine the mass of a planet, or in fact if there are multiple planets in the star's system.
A problem with this method is trying to tell apparent motion due to the atmosphere of the earth, and even instrumental defects. A possible solution is to study the stars in a cluster. It is then possible to factor out the motion that they have in common and see if there are any perturbations that could be caused by planets. Another advantage of studying clusters is that because the stars are close together it is possible to study stars at a time.
The radial velocity method works as follows: If there is a planet, the star and the planet will orbit a common centre of mass for the system. By looking at the spectrum of the star, it is possible to determine whether the star is moving towards or away from us, using the Doppler effect. The spectral lines of the star will be shifted toward the blue end of the spectrum if the star is coming toward us, and toward the red end if it is moving away. If the star periodically shifts from moving toward then away from the Earth, this will be because of an unseen companion, which is causing the system to orbit a common centre of mass. However, because the size of the planet is very small relative to the size of the star, these shifts are small. Nonetheless this is the most common method, which has been used up to now to detect extrasolar planets, and it is thought that all of the brown dwarves near to the sun have been detected mainly using this method. This method has a bias toward larger planets, of several times the mass of Jupiter.
Another indirect method is ground-based photometry- watching for occultations of stars by companion planets. When a planet passes in front of a star, the star's spectrum will change. By taking spectra of a star over a period of time it is possible to monitor to see if there are any changes that could be caused by planets. The first reported detection of a planet using this method took place earlier this year, using the ELODIE spectrograph. D. Queloz et al. detected distortions in stellar lines of the star HD209458.
Planets in the solar system don't glow; yet we can see them because they reflect light from the sun. Similarly, planets in other systems won't glow, but will reflect the light of their primary star. Looking for planets directly hasn't been used much in the past because a planet has to be very big, reflecting a lot of light, before it can be seen as different to the star that it orbits. The light of the star is many, many times brighter than the light reflected by the planet, by about a billion times. However there have been recent advances allowing direct detection of planets to be possible in the very near future. One approach is to use a coronagraph, an instrument that blocks out most of the light of the star, so that only the corona can be seen. Because there isn't the glare from the primary star present, it would then be possible to see planets in orbit. Practically, a large, ground based telescope making use of adaptive optics to overcome the problems of seeing should be able to see planets of around the size of Jupiter orbiting other stars.
Another way of refining this approach is to look at infrared rather than visual wavelengths. While a star may be a billion times brighter than its companion planet in the visual range of the electromagnetic spectrum, in the infrared the planet radiates relatively more heat. At infrared wavelengths, the planet can be only a million times less bright than the star. For example using the Keck interferometer in Hawaii hot, Jupiter like planets have already been detected close to their primary star. It is also possible to arrange the interferometer so that light from the star destructively interferes with itself, making it easier to observe planets and dust around the star.
The future for extrasolar planet searches is exciting. There are major projects planned for the beginning of the 21st century by both NASA and ESA
The Next Generation Space Telescope (NGST) is planned to replace the Hubble Space Telescope. It will have an 8-metre mirror, and will therefore be able to resolve a lot better than Hubble can. Space telescopes have the advantage that they are above perturbations caused by the Earth's atmosphere, so they do not need to be used in combination with adaptive optics, and can get a better resolution than a ground-based telescope. One of NGST's science goals is to study the formation of planetary systems.
NASA's Origins mission also plans to try to detect earth-sized planets, both indirectly with the use of the Space Interferometery Mission, and directly for stars within 15 parsecs of the sun using the Terrestrial Planet Finder (TPF). TPF will look for planets that are 'twins' of earth. TPF will use spectroscopy to determine whether or not planets have the same atmospheric composition as Earth.
The GAIA will be run by the European Space Agency. It will map the sky, determining the positions of stars, and will also be able to see larger extrasolar planets up to 500 parsecs away. Another ESA mission will be Eddington. Eddington will be a 1.2 metre space telescope, which has similar goals to the TPF. Its main mission will be to look at planets in the habitable zone around stars, which is where water can exist in liquid form. Eddington will use photometry to detect transits of a planet across a star.
BIBLIOGRAPHY
http://exoplanets.org/index.html - The Search for Extrasolar Planets, run by he Department of Astronomy at UC Berkeley
http://xxx.lpthe.jussieu.fr/abs/astro-ph/0006213 - Detection of a spectroscopic transit by the planet orbiting the star HD209458 by D. Queloz et al.
http://www.public.asu.edu/~sciref/exoplnt.htm The Search for the Extrasolar Planets: A Brief History of the Search, the Findings and the Future Implications, by George H. Bell, Arizona State University.
http://www.obs-nice.fr/baudoz/coro/coro.html Achromatic Interferential Coronagraph, Observatory of la Cote d'Azur
http://huey.jpl.nasa.gov/keck/ - The Keck Interferometer website
http://www.ngst.stsci.edu/science/ - The Next Generation Space Telescope Science Goals, from the Space Telescope Science Institute.
http://astro.estec.esa.nl/GAIA/index.html - The GAIA homepage
