essay done!
Dec. 11th, 2002 03:53 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
kbkhooray, the last big piece of work of the semester.
currently printing it off. it's shitty, but it's done.
I know I said this would be fic-centric, but this is important because now it's done I'll have time to write, or something.
hey, have a look - I'll cut the refs (which took me all bloody afternoon to sort out - evil things) and the one diagram that was basically pointless.
Introduction
The search for extra-solar planets is one of the areas of astrophysics with the greatest potential to grab the imagination. But with the stars themselves mere pinpricks in the sky, how have astronomers managed to detect over a hundred comparatively tiny planets, and how can we hope to find more? While all of the planets known at the present time have been detected using the radial velocity method, there are other techniques which could allow us to find still more, and in different areas. It may even be possible to lower the threshold of detection sufficiently to find Earth-like planets instead of the giants that we are currently restricted to. With all the planets that are being found, the problem of classification arises; even within our own system, the general term planet can be used for a wide range of sizes and compositions. A more precise classification could help with the models of planetary formation currently under debate and thus increase our understanding of the process which birthed the Earth and eventually humanity.
Methods of detection
The most successful method of planet detection so far is that of measuring the radial velocity of the star. A star with a planet is a two-body system and thus orbits around the centre of mass, producing a "wobble" in the star's motion which shows up as a sinusoidal variation in the Doppler shift of the spectrum. This shift can be measured by spectroscopy, plotting the variations in the observed spectrum to show the alternate red and blue shift of the star.
There are several programs in place taking the spectra of the stars in the attempt to find yet more planets. These include Coralie and Elodie of the Geneva Extrasolar Planet Search Programmes, two high-resolution echelle spectrometers placed in the two hemispheres in order to survey as much of the sky as possible. These spectrometers work by collimating the light from a star and using a blazed grating to interfere the individual waves to give an intense reflection of particular wavelengths. This is then focussed by another grating onto a highly sensitive CCD. Another example is the European Southern Observatory's Coude Echelle Spectrometer Planet Search Programme, which has so far discovered two planets in the course of an intensive study of forty solar-type stars.
While the above is the only method of exoplanet detection which has so far been successful, several other methods have been proposed, and some of these are currently being tested. It is possible that in the future the following techniques can be added to those available to astronomers in detecting extrasolar planets and discovering their characteristics.
If a system is inclined at close to ninety degrees, there is the possibility of the planet crossing in front of the star and causing a dip in the light, as can be observed on occasion in our system i.e. when Mercury or Venus transit the Sun. This is done by photometry, measuring the luminosity of the individual star. Thus far, it has been a case of observing planetary systems previously discovered by radial velocity measurements - taking the velocity curves from the results already obtained, the times of likely transits can be calculated and the star can be observed at those particular times. This has been successfully attempted with the star HD209458 where the luminosity was found to drop by 0.017 magnitudes at the predicted time. Further calculations showed the planet to be at an orbit of 1.27 RJ and an inclination of 87.1 degrees. In the future, it may be possible to detect planets through this method alone, though it would require the allocation of substantial observing time.
Precise astrometric observation of the stars could show the "wobble" of a star as a small proper motion on the sky. This can be done from the ground and from space. Ground-based astrometry has some difficulties, including the need for sophisticated adaptive optics to overcome the blurring caused by the atmosphere, but it is possible that the small motions could be detected using interferometry. This uses multiple telescopes and combines the results, making them the equivalent of a much larger telescope. The Very Large Array in New Mexico uses this principle for radio astronomy, electronically combining data from 27 antenna. If the detection of the variations could be achieved, conclusions could be drawn about the planets in much the same manner as for the radial velocity searches.
Space-based astrometry has the advantage of being free of the atmosphere and as such having a greater accuracy and a much smaller angular resolution. Also, the orbit of the telescope can be set so that permanent observation of certain targets is possible. There are several plans in this area, including the FAME project (Full-sky Astrometric Mapping Explorer), a survey mission for which funding is currently uncertain. The proposed telescope would survey two fields of view, separated by a fixed angle, and rotate to eventually record the entire sky, with an accuracy of approximately fifty microarcseconds. This would pave the way for pointed missions such as NASA's Space Interferometry Mission and the European Space Agency's GAIA. GAIA will follow the already proven template of the Hipparcos mission, which has successfully recorded information on several thousand stars, but be a refined version with much higher accuracy.
Further missions such as the ESA's Darwin could make it possible to actually see planets at mid-infrared wavelengths by using several small telescopes linked together as a nulling interferometer. It would be stationed in a solar orbit at the second Lagrangian point, a point of equilibrium 1.5 million kilometres outside the Earth's orbit. This offers uninterrupted observations, as the Sun, Earth and Moon are always behind it. Looking at a star with these telescopes and using a slight delay would give interference fringes, and these could be rotated to leave the star itself in a null position and allow the planets to show. A similar mission that is currently at the proposal stage is the Terrestrial Planet Finder, funded by NASA. It is yet to be decided whether this would take the form of an interferometer or a coronagraph, but it should be able to do such things as take the spectra of giant planets orbiting other stars, information impossible to extrapolate from other sources. Also, the chance to actually see a planet orbiting another star is one that would surely excite anyone.
Another direct method of observation detects the magnetic emission from a planet, particularly cyclotron-maser radiation which is strong enough to be detected at Galactic distances. Of course, the star emits radiation also, but the emission from the planet is modulated thanks to its rotation and can thus be distinguished. The period of the modulation gives the rotation rate of the planet, as it does for the planets in our system with differential rotation such as Jupiter. Also, any distortions of the curve could show us satellites of the planet, and of course we can find the maximum strength of the magnetic field of the planet. This is particularly exciting because some theorists believe that the magnetic field may, in blocking stellar radiation, make it possible for life to exist.
Some observations have been done on previously discovered planets using the Very Large Array in certain frequency bands, and while no radiation was detected, it is possible that the frequencies observed were not those of emission, or that the emission from the planet was simply too weak to be detected at this distance. Further observations with enhanced equipment will have to be conducted in order to verify the efficacy of this technique.
The final technique is microlensing, currently most associated with the search for Dark Matter. Einstein's theory of Relativity tells us that light from a distant star can be warped by an object halfway to the source, magnifying the image. If the lensing object is a two-body system, the light curve of the lensed object will be warped, being asymmetric or showing sharp points. A planet orbiting the lens would lead to a short-term "wiggle". These curves could be analysed to give us substantial amounts of information about the system. However, only one in twenty lensing events are of the high magnification required to show the fine structure necessary for calculation.
As there are currently several searches for MACHOs utilising this technology, it is a simple matter to piggyback the search for extrasolar planets onto these programs. With a lensing event found, if there were a network of telescopes in place, the entire curve (which would last several hours) could be recorded and subsequently analysed. The probability of detecting a planet similar to Jupiter orbiting the lensing object has been calculated to be 17%, which is quite high. While observations would span the Galaxy, the main disadvantage of this technique is that repeat observations of the lensing are highly unlikely to be possible, and so confirmation of the result will be impossible.
Classification
The first major difficulty in classifying planets is deciding what precisely falls under that general heading. "Planet" is generally accepted to mean an object which orbits a star and is too small to support nuclear reactions in itself, usually with a mass less than ten or twenty Jovian masses. However, this can include such objects as brown dwarfs ("failed stars") which would skew the results. We already have a distinction between terrestrial planets and gas giants in our own system, the former being rockier, denser and closer in. However, since only planets with masses comparable to Jupiter are currently detectable, this is of no use in organising extra-solar planets.
Suggested refinements of the definition have included the sub-divisions of "eccentric planets" and "Î-Ret-like planets". The former would be gas giants with orbits of an eccentricity greater than 0.2, while the latter (named for epsilon Reticulum, around which the first such exoplanet was discovered) are gas giants with low-eccentricity orbits similar to those in our own solar system. It has been suggested that the latter type are more likely to occur around high-metallicity stars, as this applies to all but one of those detected. It is an interesting speculation, but difficult to explain, and with the small number of such planets known. This is another potential boost to discovery - with the high-metal stars singled out, the likelihood of detecting such a planet ought to increase.
Conclusion
In conclusion, while there are several potential methods of detecting planets orbiting other stars, only one has thus far been effective. The measurement of radial velocity been used in the discovery of over a hundred planets, and will likely be used for many more. Other methods such as astrometry, microlensing and observation of transits have their advantages and their disadvantages, and will reveal different properties of the planet observed, but this will be of no matter if they never actually work. Perhaps these methods should be reserved for follow-up observations on planets already discovered by spectroscopy and the like.
As to classification, the above empirical classifications, while interesting and potentially convenient, will be of no benefit to astronomy unless they prove to have other correlations such as mass and composition, which would suggest a similarity in the formation of each body.
wasn't that exciting?
wah. hungry. and my drugs haven't started working yet.
currently printing it off. it's shitty, but it's done.
I know I said this would be fic-centric, but this is important because now it's done I'll have time to write, or something.
hey, have a look - I'll cut the refs (which took me all bloody afternoon to sort out - evil things) and the one diagram that was basically pointless.
Introduction
The search for extra-solar planets is one of the areas of astrophysics with the greatest potential to grab the imagination. But with the stars themselves mere pinpricks in the sky, how have astronomers managed to detect over a hundred comparatively tiny planets, and how can we hope to find more? While all of the planets known at the present time have been detected using the radial velocity method, there are other techniques which could allow us to find still more, and in different areas. It may even be possible to lower the threshold of detection sufficiently to find Earth-like planets instead of the giants that we are currently restricted to. With all the planets that are being found, the problem of classification arises; even within our own system, the general term planet can be used for a wide range of sizes and compositions. A more precise classification could help with the models of planetary formation currently under debate and thus increase our understanding of the process which birthed the Earth and eventually humanity.
Methods of detection
The most successful method of planet detection so far is that of measuring the radial velocity of the star. A star with a planet is a two-body system and thus orbits around the centre of mass, producing a "wobble" in the star's motion which shows up as a sinusoidal variation in the Doppler shift of the spectrum. This shift can be measured by spectroscopy, plotting the variations in the observed spectrum to show the alternate red and blue shift of the star.
There are several programs in place taking the spectra of the stars in the attempt to find yet more planets. These include Coralie and Elodie of the Geneva Extrasolar Planet Search Programmes, two high-resolution echelle spectrometers placed in the two hemispheres in order to survey as much of the sky as possible. These spectrometers work by collimating the light from a star and using a blazed grating to interfere the individual waves to give an intense reflection of particular wavelengths. This is then focussed by another grating onto a highly sensitive CCD. Another example is the European Southern Observatory's Coude Echelle Spectrometer Planet Search Programme, which has so far discovered two planets in the course of an intensive study of forty solar-type stars.
While the above is the only method of exoplanet detection which has so far been successful, several other methods have been proposed, and some of these are currently being tested. It is possible that in the future the following techniques can be added to those available to astronomers in detecting extrasolar planets and discovering their characteristics.
If a system is inclined at close to ninety degrees, there is the possibility of the planet crossing in front of the star and causing a dip in the light, as can be observed on occasion in our system i.e. when Mercury or Venus transit the Sun. This is done by photometry, measuring the luminosity of the individual star. Thus far, it has been a case of observing planetary systems previously discovered by radial velocity measurements - taking the velocity curves from the results already obtained, the times of likely transits can be calculated and the star can be observed at those particular times. This has been successfully attempted with the star HD209458 where the luminosity was found to drop by 0.017 magnitudes at the predicted time. Further calculations showed the planet to be at an orbit of 1.27 RJ and an inclination of 87.1 degrees. In the future, it may be possible to detect planets through this method alone, though it would require the allocation of substantial observing time.
Precise astrometric observation of the stars could show the "wobble" of a star as a small proper motion on the sky. This can be done from the ground and from space. Ground-based astrometry has some difficulties, including the need for sophisticated adaptive optics to overcome the blurring caused by the atmosphere, but it is possible that the small motions could be detected using interferometry. This uses multiple telescopes and combines the results, making them the equivalent of a much larger telescope. The Very Large Array in New Mexico uses this principle for radio astronomy, electronically combining data from 27 antenna. If the detection of the variations could be achieved, conclusions could be drawn about the planets in much the same manner as for the radial velocity searches.
Space-based astrometry has the advantage of being free of the atmosphere and as such having a greater accuracy and a much smaller angular resolution. Also, the orbit of the telescope can be set so that permanent observation of certain targets is possible. There are several plans in this area, including the FAME project (Full-sky Astrometric Mapping Explorer), a survey mission for which funding is currently uncertain. The proposed telescope would survey two fields of view, separated by a fixed angle, and rotate to eventually record the entire sky, with an accuracy of approximately fifty microarcseconds. This would pave the way for pointed missions such as NASA's Space Interferometry Mission and the European Space Agency's GAIA. GAIA will follow the already proven template of the Hipparcos mission, which has successfully recorded information on several thousand stars, but be a refined version with much higher accuracy.
Further missions such as the ESA's Darwin could make it possible to actually see planets at mid-infrared wavelengths by using several small telescopes linked together as a nulling interferometer. It would be stationed in a solar orbit at the second Lagrangian point, a point of equilibrium 1.5 million kilometres outside the Earth's orbit. This offers uninterrupted observations, as the Sun, Earth and Moon are always behind it. Looking at a star with these telescopes and using a slight delay would give interference fringes, and these could be rotated to leave the star itself in a null position and allow the planets to show. A similar mission that is currently at the proposal stage is the Terrestrial Planet Finder, funded by NASA. It is yet to be decided whether this would take the form of an interferometer or a coronagraph, but it should be able to do such things as take the spectra of giant planets orbiting other stars, information impossible to extrapolate from other sources. Also, the chance to actually see a planet orbiting another star is one that would surely excite anyone.
Another direct method of observation detects the magnetic emission from a planet, particularly cyclotron-maser radiation which is strong enough to be detected at Galactic distances. Of course, the star emits radiation also, but the emission from the planet is modulated thanks to its rotation and can thus be distinguished. The period of the modulation gives the rotation rate of the planet, as it does for the planets in our system with differential rotation such as Jupiter. Also, any distortions of the curve could show us satellites of the planet, and of course we can find the maximum strength of the magnetic field of the planet. This is particularly exciting because some theorists believe that the magnetic field may, in blocking stellar radiation, make it possible for life to exist.
Some observations have been done on previously discovered planets using the Very Large Array in certain frequency bands, and while no radiation was detected, it is possible that the frequencies observed were not those of emission, or that the emission from the planet was simply too weak to be detected at this distance. Further observations with enhanced equipment will have to be conducted in order to verify the efficacy of this technique.
The final technique is microlensing, currently most associated with the search for Dark Matter. Einstein's theory of Relativity tells us that light from a distant star can be warped by an object halfway to the source, magnifying the image. If the lensing object is a two-body system, the light curve of the lensed object will be warped, being asymmetric or showing sharp points. A planet orbiting the lens would lead to a short-term "wiggle". These curves could be analysed to give us substantial amounts of information about the system. However, only one in twenty lensing events are of the high magnification required to show the fine structure necessary for calculation.
As there are currently several searches for MACHOs utilising this technology, it is a simple matter to piggyback the search for extrasolar planets onto these programs. With a lensing event found, if there were a network of telescopes in place, the entire curve (which would last several hours) could be recorded and subsequently analysed. The probability of detecting a planet similar to Jupiter orbiting the lensing object has been calculated to be 17%, which is quite high. While observations would span the Galaxy, the main disadvantage of this technique is that repeat observations of the lensing are highly unlikely to be possible, and so confirmation of the result will be impossible.
Classification
The first major difficulty in classifying planets is deciding what precisely falls under that general heading. "Planet" is generally accepted to mean an object which orbits a star and is too small to support nuclear reactions in itself, usually with a mass less than ten or twenty Jovian masses. However, this can include such objects as brown dwarfs ("failed stars") which would skew the results. We already have a distinction between terrestrial planets and gas giants in our own system, the former being rockier, denser and closer in. However, since only planets with masses comparable to Jupiter are currently detectable, this is of no use in organising extra-solar planets.
Suggested refinements of the definition have included the sub-divisions of "eccentric planets" and "Î-Ret-like planets". The former would be gas giants with orbits of an eccentricity greater than 0.2, while the latter (named for epsilon Reticulum, around which the first such exoplanet was discovered) are gas giants with low-eccentricity orbits similar to those in our own solar system. It has been suggested that the latter type are more likely to occur around high-metallicity stars, as this applies to all but one of those detected. It is an interesting speculation, but difficult to explain, and with the small number of such planets known. This is another potential boost to discovery - with the high-metal stars singled out, the likelihood of detecting such a planet ought to increase.
Conclusion
In conclusion, while there are several potential methods of detecting planets orbiting other stars, only one has thus far been effective. The measurement of radial velocity been used in the discovery of over a hundred planets, and will likely be used for many more. Other methods such as astrometry, microlensing and observation of transits have their advantages and their disadvantages, and will reveal different properties of the planet observed, but this will be of no matter if they never actually work. Perhaps these methods should be reserved for follow-up observations on planets already discovered by spectroscopy and the like.
As to classification, the above empirical classifications, while interesting and potentially convenient, will be of no benefit to astronomy unless they prove to have other correlations such as mass and composition, which would suggest a similarity in the formation of each body.
wasn't that exciting?
wah. hungry. and my drugs haven't started working yet.
![[identity profile]](https://www.dreamwidth.org/img/silk/identity/openid.png){kind=small}
MACHO Macho man...woman...whatever...
Date: 2002-12-11 06:24 pm (UTC)As there are currently several searches for MACHOs
Interesting read. Really. :)
Re: MACHO Macho man...woman...whatever...
Date: 2002-12-11 07:47 pm (UTC)sophisticated senses of humour in action...
thank you for reading it!
no subject
Date: 2002-12-22 08:58 pm (UTC)I didn't understand some of it, but then I don't do astro so wouldn't expect to. If you're wondering why I read it. It's because there's nothing else to do. It's almost five am, but I'm not tired yet so I thought I'd read.
Sigh
Have a groovy christmas...
Rev