Exoplanet
From RationalWiki
An exoplanet is a planet outside of our solar system.
These planets orbit stars in other star systems. Our sun is a star, and all the other stars are also suns. They look tiny compared to our sun because they are extremely far away. "Rogue planets" (planets drifting through deep space instead of describing an orbit around a star) are also theoretically possible.
The first exoplanet discovered was PSR B1257+12 during a 1993 study led by Alex Wolszczan of Pennsylvania State University.[1]
As of 2010, approximately 400 exoplanets, scattered over dozens of star systems, have been found. Most of these exoplanets are gas giants; this is most likely due to the limits of current detection methods.
Some rocky planets have been found as well. Unlike gas giants, rocky planets may, given the right conditions, be able to sustain life.[2]
Currently the most notable of these rocky planets is the relatively nearby (20 light-years) planet Gliese 581c. Of all the known exoplanets, this one comes closest to being capable of sustaining life.[3]
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[edit] Detection Methods
[edit] Astrometric Method
Exoplanets can be found using astrometry.[4]
When a planet orbits a star, these objects exert a mutual gravitational force on each other, causing the system to revolve around the system's center of gravity.
Because of the star's larger mass, the system's center of gravity will lie close to the star's core. While it is usually very small, the star does describe an actual orbit.
Since most star systems in the galaxy lie in approximately the same plane, an observer on Earth would observe a "wobbling" movement of the star, indicating the presence of a planet.
Advantages:
- Relatively simple, but has no real advantages.
Drawbacks:
- If there are more than 3 planets in the system, it is impossible to derive information (radius of orbit, mass, period of orbit) on the individual planets. (this is true for all methods, except the transit method, however the crosschecking of different methods can be used to derive these data.)
- Only the best telescopes are capable of detecting the minute wobbling movements of the star.
[edit] Radial Velocity Method
The radial velocity method [5] (also known as the Doppler method) uses the same basic as the astrometric method. Instead of directly observing the star's wobbling, this technique uses the Doppler Effect to measure differences in the radial velocity of a star (how fast the star is moving towards, or away) relative to Earth.
Advantages:
- This method is accurate, and is available to many observatories. It has been the most productive of exoplanet detection methods so far.
Drawbacks:
- The range of this technique is limited to 160 light-years.
- Some planets describe orbits almost perpendicular to that of Earth, these are often difficult, if not impossible to detect using the radial velocity method.
[edit] Transit Method
The transit method [6] works by observing the intensity of the light from a star. When a planet moves in front of the star (relative to Earth), the light we detect from the star will be slightly dimmed — even if the planet itself is not visible, the dimming of the starlight can still be detected.
Sometimes the planet can be directly observed by a telescope.
Advantages:
- The transit method also allows calculation of the planet's size (if the star's radius is known) and may even reveal details about the planet's atmosphere through spectroscopy.
- This method can also be used obtain information on individual planets when crosschecked with the radial velocity or astrometric method.
Drawbacks:
- There have been false readings: verification by the radial velocity method is required.
- The transit method only works when the planet's orbit lies in the same plane as Earth's orbit and often the star has to be monitored for years before a transit occurs.
[edit] Gravitational Microlensing Method
The gravitational microlensing method[7] draws on Einstein's theory of General Relativity: what we perceive as gravitation is actually the folding of space near a massive object, meaning light, consisting of massless photons, is still influenced by gravity
The gravitational field of a star acts as a magnifying glass on a background star, making the background appear to shift or change in size, if the mass of the lens star is known, the contribution to the lensing by a planet orbiting the lensing star can be calculated.
Advantages:
- The range of this method is several thousands of parsecs (1 parsec is 3.26 light-years).
- This method is precise enough to detect rocky planets
Drawbacks:
- This method requires a rare near perfect alignment of earth, the lensing star, and the background star.
[edit] Example
Epsilon Eridani is a star system 10.5 Light Years away from Earth. It is a red dwarf star approximately 850 million years old. There are currently two confirmed planets in the system: A Jupiter sized planet around outside of the first asteroid belt and a Saturn sized planet around the same distance to the star as Pluto. It is expected that more planets will be found in the system within a decade when there are better telescopes.
[edit] External links to sources
- California & Carnegie Planet Search
- Encyclopedia of Astrobiology, Astronomy, and Spaceflight, David Darling
- The Extrasolar Planets Encyclopaedia
[edit] Footnotes
- ↑ Earth-Sized Planets Confirmed, But They're Dead Worlds SPACE.com, Robert Roy Britt [1]
- ↑ This point is highly debateable; there is no real reason why some form of life could not exist elsewhere than rocky planets.
- ↑ Again, we are guilty of terracentrism here, imagining that only life like ours on a planet like ours could occur.
- ↑ Astrometry: The Past and Future of Planet Hunting The Planetary Society [2]
- ↑ Radial Velocity Method The Encyclopedia f Astrobiology, Astronomy and Spaceflight, David Darling [3]
- ↑ Detecting Other Worlds: The Photometric Transit or 'Wink' Method Seti.org [4]
- ↑ Gravitational Microlensing Microlensing Observations in Astrophysics [5]
