There are several methods used to detect and measure extrasolar planets, each with its own strengths and limitations:
1. Radial Velocity Method (Doppler Spectroscopy):
* Principle: Detects the wobble of a star caused by the gravitational pull of an orbiting planet.
* How it works: Measures the shift in the star's spectral lines due to the Doppler effect.
* Strengths: Can detect planets with relatively small masses, particularly those in close orbits.
* Limitations: Requires high-precision measurements and can be affected by stellar activity (sunspots, flares).
* Examples: Discovery of 51 Pegasi b, the first confirmed exoplanet.
2. Transit Method:
* Principle: Detects the slight dimming of a star's light as a planet passes in front of it.
* How it works: Measures the change in brightness over time.
* Strengths: Can detect planets of different sizes, including those in wide orbits.
* Limitations: Requires the planet's orbit to be edge-on to our line of sight, limited to detecting planets that transit.
* Examples: Discovery of Kepler-186f, the first Earth-sized planet in the habitable zone of another star.
3. Astrometry:
* Principle: Detects the wobble of a star caused by an orbiting planet by measuring its position in the sky over time.
* How it works: Measures the change in the star's proper motion and parallax.
* Strengths: Can detect planets of various sizes, including those in distant orbits.
* Limitations: Requires very precise measurements and is challenging due to the small stellar motions involved.
* Examples: Limited successful detections due to technical difficulties, but promising for future space telescopes.
4. Direct Imaging:
* Principle: Directly observing the faint light emitted or reflected by an exoplanet.
* How it works: Using specialized telescopes and instruments to block out the star's light.
* Strengths: Provides direct information about the planet's atmosphere, temperature, and composition.
* Limitations: Requires the planet to be large, young, and far from its star, limiting the number of detectable planets.
* Examples: Imaged planets like HR 8799 b, c, d, and e.
5. Microlensing:
* Principle: Detects the gravitational lensing effect of a planet, magnifying the light of a distant star.
* How it works: Measures the brightening of a background star as a planet passes in front of it.
* Strengths: Can detect planets of various sizes, including those in wide orbits.
* Limitations: Events are rare and short-lived, making it challenging to observe.
* Examples: Discovery of OGLE-2005-BLG-390Lb, the first planet detected by microlensing.
6. Timing Variations:
* Principle: Detects the wobble of a pulsar's timing caused by the gravitational pull of an orbiting planet.
* How it works: Measures the precise timing of pulses emitted by pulsars.
* Strengths: Can detect planets with relatively small masses, particularly those in close orbits.
* Limitations: Limited to planets orbiting pulsars, a specific type of star.
* Examples: Discovery of PSR B1257+12 b, c, and d, the first planets discovered around a pulsar.
Measuring Exoplanet Properties:
These methods not only detect exoplanets but also provide information about their:
* Mass: Derived from the radial velocity and timing variations methods.
* Radius: Determined from the transit and direct imaging methods.
* Orbital period: Determined from all methods.
* Orbital eccentricity: Measured using the radial velocity method.
* Density: Calculated from the mass and radius.
* Atmospheric composition: Analyzed from the light reflected or emitted by the planet.
* Temperature: Inferred from the planet's distance from its star and its atmospheric properties.
These methods continue to improve, leading to the discovery and characterization of an increasing number of exoplanets, providing insights into the diversity of planetary systems beyond our own.
