What is an extrasolar planet?
An extrasolar planet, or
exoplanet, is a planet outside the
Solar System. A total of 763 such planets (in 611 planetary systems and 101
multiple planetary systems) have been identified as of April 14, 2012.
Estimates of the frequency of systems strongly suggest that more than
50% of Sun-like
stars harbor at least one planet.
In a 2012 study, each star of the 100 billion or so in our Milky Way
galaxy is estimated to host "on average ... at least 1.6 planets."
Accordingly, at least 160 billion star-bound planets may exist in the
Milky Way Galaxy alone.
Unbound free-floating planetary-mass bodies in the Milky Way may number
in the trillions with 100,000 objects larger than Pluto for every
For centuries, many philosophers and
scientists supposed that extrasolar planets existed, but there was no
way of knowing how common they were or how similar they might be to the
planets of the Solar System. Various detection claims made starting in
the nineteenth century were all eventually rejected by astronomers. The
first confirmed detection came in 1992, with the
discovery of several terrestrial-mass planets orbiting the pulsar PSR
The first confirmed detection of an exoplanet orbiting a main-sequence star was made
in 1995, when a giant planet was found in a four-day orbit around the
nearby star 51 Pegasi. Due to improved observational techniques, the
rate of detections has increased rapidly since then. Some
exoplanets have been directly imaged by telescopes, but the vast
majority have been detected through indirect methods such as radial
Most known exoplanets are giant planets
believed to resemble Jupiter or Neptune. That reflects a sampling bias,
since massive planets are much easier to observe.
Some relatively lightweight exoplanets, only a few times more massive
than Earth (now known by the term Super-Earth), are known as well;
statistical studies now indicate that they actually outnumber giant
while recent discoveries have included Earth-sized and smaller planets
and a handful that appear to exhibit other Earth-like properties.
There also exist planetary-mass objects that orbit brown dwarfs, and there
exist others that "float free" in space not bound to any star, however
the term "planet" isn't always applied to these objects.
The discovery of extrasolar planets, some of
which orbit in the habitable zone where life as we know it can exist,
have intensified interest in the search for extraterrestrial life.
Thus, the search for extrasolar planets also includes the study of
planetary habitability, which considers a wide range of factors in
determining an extrasolar planet's suitability for hosting life.