Extrasolar Planets
Extrasolar Planets
| Group Name | expl |
| Reference | The Extrasolar Planets Encyclopaedia
Jean Schneider (CNRS-LUTH, Paris Observatory) |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | Yes |
| Files | expl.speck, expl.label |
| Dependencies | target-blue.sgi |
| Census | 256 planets in 219 systems |
Extrasolar planets, or exoplanets, are a relatively new phenomenon in astronomy. While many astronomers believed in their existence, no observational evidence was available until 1995. Since then, we have discovered more than 200 systems consisting of one or more planets around a host star.
These planets were not discovered in the traditional sense of past planetary discoveries; they are so far away and so small that we cannot actually see them. Instead, for many of the systems, the host star's spectrum is analyzed, and an extremely small motion is inferred for the star. This tiny motion is due to the orbiting planet.
While we think of the Sun as being stationary, it actually moves, or wobbles, because of the planets that orbit it. The larger the planet, the larger the wobble. This is because the center of the orbit is actually located at a point called the “center of mass” of the system. So, for example, the Sun-Jupiter system's center of mass is more than 778,000 kilometers (483,000 miles) from the Sun's center. This point, along the line connecting the two bodies, lies just outside the Sun's photosphere, or “surface,” which has a radius of about 696,000 km (432,000 miles). While we do not perceive it, the Sun is orbiting this point and would be observed to wobble from a point of view outside the Solar System.
Many of the exoplanets were discovered with this observational technique. Other techniques include pulsar timings, measuring the periodic variation in the light arrival time; transit photometry, which measures the periodic variation in the light of the host star; and gravitational microlensing.
Planetary Hosts
Most planetary systems are hosted by main-sequence stars. These systems take the names of their host star. Some of these stars have Greek names, like Upsilon Andromedae; some have Flamsteed names, like 51 Pegasi, the first system detected in 1995; and others have HD numbers from the Henry Draper star catalog. Each have the lowercase lettered name of the planet or planets in the system. The first planet is named “b,” and subsequent planets are given letters in sequence. The two systems with the most planets are 55 Cancri which has 5 planets and HD 160691, which has 4 planets.
Two pulsars have been found to host planets and are labeled with “PSR.” The detection method used for pulsars allows their planets to be found at greater distances. The two systems in the atlas are 978 and 12,388 light-years from Earth. Compare that with the main-sequence-star hosts that are within 350 light-years (most are within 150 light-years).
The remaining handful were found by microlensing and transit surveys. These are considerably farther, ranging from 500 to 20,000 light-years from Earth.
The systems are represented by a blue target around each host star. This image is not meant to represent the orbit of the planet; it is simply a marker.
The extrasolar planets have a number of data variables to offer flexibility when visualizing these data.
| Data Variables for the Extrasolar Planets | |||
| Number | Name | Description | Units |
| 0 | numplanets | Number of planets in the system | -- |
| 1 | discovered | Year of discovery of the first planet in the system | years |
| 2 | distly | Distance to the system | light-years |
| 3 | texnum | Texture number | -- |
You can use these variables in conjunction with the thresh command to see how many systems have more than one planet (thresh numplanets > 2), or how many planets were discovered between 1995 and 2000 (thresh discovered 1995 2000).
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott