RigelKentaurusA
New Member
There doesn't seem to be an extrasolar planets thread, so I'll create and maintain one.
A number of extrasolar planets have been discovered since 1995 through a variety of different methods, and the total is over 500 now. Planets are discovered through two main methods.
Doppler Spectroscopy
A planet will gravitationally displace a star that it orbits, causing the star to revolve around the system's centre of mass. The period of the stellar orbit around this barycentre is equivalent to the orbital period of the planet, and the radial velocity of the star depends on the gravity (and thus mass) of the perturbing planet.
The moving star manifests itself in the periodic red- and blue-shifting of absorption lines in the star's spectrum. And as such, stars with easily measurable spectral lines are the best targets for this detection method.
Because the velocity component of the star is radial (that is, toward or away from the observer), the amount of observed stellar motion will be a lower limit to the true amount of stellar motion. Thus, there is an inclination degeneracy causing us to know only a lower limit to the mass of the planetary companion. Systems that are edge-on have inclinations of 90,° (with respect to the plane of the sky), and the true mass will be equal to the minimum mass. Systems that are face-on have inclinations of 0,° to the plane of the sky, and have no radial velocity component.
Transit photometry
In the case of an edge-on, ~90,° inclined system, the planet will orbit around its star such that it passes between it's star and Earth, resulting in the blocking of some starlight (an event known as a transit). The amount of starlight blocked is a function of the square of the ratio of their radii, and thus you can determine the radius of the planet by how much the star dims.
Equipped with the radius and mass from Doppler spectroscopy, the average density is easily measured, placing constraints on the interior structure of the planet. Also, a transiting system has its inclination known, and so the true mass of the planet is also known. Mass measurements are required to confirm planets discovered by transit photometry, as many other astrophysical phenomena can masquerade as transits.
The transparent outer layers of a planet with an atmosphere will affect the spectrum of the starlight by absorbing more wavelengths. As such, the spectrum of the star will appear to change slightly during a transit event as starlight is filtered through the atmosphere. The spectrum of the planet's atmosphere is thereby measured by subtracting the stellar spectrum during the transit from the stellar spectrum out of transit.
Astrometry
Highly accurate telescopes can actually observe the barycentric motion of the star, yielding the mass of the planet as well as the entire orbit.
Pulsation timing
A pulsating star with a constant frequency can be used to detect orbiting planets. It's radial velocity can be measured by observing the timing in between pulses as a result of the star's barycentric motion. Because light will have to travel farther to make up for the extra distance from Earth across the barycenter, the observed frequency of the pulsations will shift up and down.
Gravitational microlensing.
When one star passes in front of another, it's gravity lenses the background star's light. A planetary companion to the lensing star will also briefly amplify the background star's light.
Several groups and instruments are actively involved with searching for extrasolar planets. I will list the major ones.
HARPS
High-precision spectrograph at ESO's 3.6 metre telescope at La Silla. Has discovered most of the confirmed low-mass planets.
SOPHIE
Spectrograph at Haute-Provence Observatory. Several planets discovered, and involved in follow-up confirmation of candidates from the Kepler mission.
CoRoT
ESA spacecraft mission dedicated to asteroseismology and searching for extrasolar planet transits. Several planets detected.
Kepler
NASA spacecraft dedicated to quantifying the frequency of planets around a variety of stellar types. Over 700 candidates detected, a few have been confirmed.
SuperWASP
Ground-based transit search. Over 40 planets found.
HATnet
Ground-based transit search. Over 25 planets found.
A number of extrasolar planets have been discovered since 1995 through a variety of different methods, and the total is over 500 now. Planets are discovered through two main methods.
Doppler Spectroscopy
A planet will gravitationally displace a star that it orbits, causing the star to revolve around the system's centre of mass. The period of the stellar orbit around this barycentre is equivalent to the orbital period of the planet, and the radial velocity of the star depends on the gravity (and thus mass) of the perturbing planet.
The moving star manifests itself in the periodic red- and blue-shifting of absorption lines in the star's spectrum. And as such, stars with easily measurable spectral lines are the best targets for this detection method.
Because the velocity component of the star is radial (that is, toward or away from the observer), the amount of observed stellar motion will be a lower limit to the true amount of stellar motion. Thus, there is an inclination degeneracy causing us to know only a lower limit to the mass of the planetary companion. Systems that are edge-on have inclinations of 90,° (with respect to the plane of the sky), and the true mass will be equal to the minimum mass. Systems that are face-on have inclinations of 0,° to the plane of the sky, and have no radial velocity component.
Transit photometry
In the case of an edge-on, ~90,° inclined system, the planet will orbit around its star such that it passes between it's star and Earth, resulting in the blocking of some starlight (an event known as a transit). The amount of starlight blocked is a function of the square of the ratio of their radii, and thus you can determine the radius of the planet by how much the star dims.
Equipped with the radius and mass from Doppler spectroscopy, the average density is easily measured, placing constraints on the interior structure of the planet. Also, a transiting system has its inclination known, and so the true mass of the planet is also known. Mass measurements are required to confirm planets discovered by transit photometry, as many other astrophysical phenomena can masquerade as transits.
The transparent outer layers of a planet with an atmosphere will affect the spectrum of the starlight by absorbing more wavelengths. As such, the spectrum of the star will appear to change slightly during a transit event as starlight is filtered through the atmosphere. The spectrum of the planet's atmosphere is thereby measured by subtracting the stellar spectrum during the transit from the stellar spectrum out of transit.
Astrometry
Highly accurate telescopes can actually observe the barycentric motion of the star, yielding the mass of the planet as well as the entire orbit.
Pulsation timing
A pulsating star with a constant frequency can be used to detect orbiting planets. It's radial velocity can be measured by observing the timing in between pulses as a result of the star's barycentric motion. Because light will have to travel farther to make up for the extra distance from Earth across the barycenter, the observed frequency of the pulsations will shift up and down.
Gravitational microlensing.
When one star passes in front of another, it's gravity lenses the background star's light. A planetary companion to the lensing star will also briefly amplify the background star's light.
Several groups and instruments are actively involved with searching for extrasolar planets. I will list the major ones.
HARPS
High-precision spectrograph at ESO's 3.6 metre telescope at La Silla. Has discovered most of the confirmed low-mass planets.
SOPHIE
Spectrograph at Haute-Provence Observatory. Several planets discovered, and involved in follow-up confirmation of candidates from the Kepler mission.
CoRoT
ESA spacecraft mission dedicated to asteroseismology and searching for extrasolar planet transits. Several planets detected.
Kepler
NASA spacecraft dedicated to quantifying the frequency of planets around a variety of stellar types. Over 700 candidates detected, a few have been confirmed.
SuperWASP
Ground-based transit search. Over 40 planets found.
HATnet
Ground-based transit search. Over 25 planets found.
