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Extrasolar planets ( just the facts - a article written in 2007)
An extrasolar planet, or exoplanet, is a planet beyond the Solar System. As of March 2007, the count of known exoplanets stands at 215. The vast majority have been detected through various indirect methods rather than actual imaging. Most of them are giant planets likely to resemble Jupiter more than Earth.
Known exoplanets are members of planetary systems that orbit a star. There have also been unconfirmed reports of free-floating planetary-mass objects (sometimes called "rogue planets"): that is, ones that do not orbit any star. Since such objects do not satisfy the working definition of "planet" adopted by the International Astronomical Union, & since their existence remains unconfirmed, they will not be discussed in this article.
Extrasolar planets became a subject of scientific investigation in the mid-nineteenth century. Astronomers generally supposed that some existed, but it was a mystery how common they were & how similar they were to the planets of the Solar System. The first confirmed detections were finally made in the 1990s; since 2002, more than twenty have been discovered every year. It is now estimated that at least 10% of sunlike stars have planets, & the true proportion may be much higher. The discovery of extrasolar planets further raises the question of whether some might support extraterrestrial life.
Claims about the detection of exoplanets have been made for over
a century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855,
Capt. W. S. Jacob at the East India Company's Madras Observatory reported that
orbital anomalies made it "highly probable" that there was a "planetary
body" in this system. In the 1890s, Thomas J. J. See of the University of
Chicago & the United States Naval Observatory stated that the orbital anomalies
proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period
around one of the stars. However, Forest Ray Moulton soon published a paper proving
that a three-body system with those orbital parameters would be highly unstable.
During the 1950s & 1960s, Peter van de Kamp of Swarthmore College made another
prominent series of detection claims, this time for planets orbiting Barnard's
Star. Astronomers now generally regard all the early reports of detection as erroneous.
The first published discovery to have received subsequent confirmation was
made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, &
S. Yang. Their radial-velocity observations suggested that a planet orbited the
star Gamma Cephei (also known as Alrai). They remained cautious about claiming
a true planetary detection, & widespread skepticism persisted in the astronomical
community for several years about this & other similar observations. It was
mainly because the observations were at the very limits of instrumental capabilities
at the time. Another source of confusion was that some of the possible planets
might instead have been brown dwarfs, objects that are intermediate in mass between
planets & stars.
The following year, additional observations were published that supported the reality of the planet orbiting Gamma Cephei, though subsequent work in 1992 raised serious doubts. Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.
In 1991, Andrew Lyne, M. Bailes & S.L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations. The claim briefly received intense attention, but Lyne & his team soon retracted it.
In early 1992, radio astronomers Aleksander Wolszczan (Polish) & Dale Frail (Canadian) announced the discovery of planets around another pulsar, PSR 1257+12. This discovery was quickly confirmed, & is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova & then spiraled into their current orbits.
On October 6, 1995, Michel Mayor & Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery was made at the Observatoire de Haute-Provence & ushered in the modern era of exoplanetary discovery. Technological advances, most notably in high-resolution spectroscopy, led to the detection of many new exoplanets at a rapid rate. These advances allowed astronomers to detect exoplanets indirectly by measuring their gravitational influence on the motion of their parent stars. Several extrasolar planets were eventually also detected by observing the variation in a star's apparent luminosity as a planet passed in front of it.
To date, 212 exoplanets have been found, including a few that were confirmations of controversial claims from the late 1980s. Many of these discoveries were made by a team led by Geoffrey Marcy & R. Paul Butler at the University of California's Lick & Keck Observatories. The first system to have more than one planet detected was ? Andromedae. Twenty such multiple-planet systems are now known. Among the known exoplanets are four pulsar planets orbiting two separate pulsars. Infrared observations of circumstellar dust disks also suggest the existence of millions of comets in several extrasolar systems.
Methods of detecting extrasolar planets
Planets are extremely
faint light sources compared to their parent stars. At visible wavelengths, they
usually have less than a millionth of their parent star's brightness. In addition
to the intrinsic difficulty of detecting such a faint light source, the parent
star causes a glare that washes it out.
For those reasons, current telescopes can only directly image exoplanets under exceptional circumstances. Specifically, it may be possible when the planet is especially large (considerably larger than Jupiter), widely separated from its parent star, & young (so that it is hot & emits intense infrared radiation).
The vast majority of known extrasolar planets have been discovered through indirect methods. At the present time, six indirect methods have yielded success:
Astrometry consists of precisely
measuring a star's position in the sky & observing the ways in which that
position changes over time. If the star has a planet, then the gravitational influence
of the planet will cause the star itself to move in a tiny circular or elliptical
orbit about their common center of mass.
Radial velocity: This is also known
as the "Doppler method". Variations in the speed with which the star
moves towards or away from Earth — that is, variations in the radial velocity
of the star with respect to Earth — can be deduced from the displacement
in the parent star's spectral lines due to the Doppler effect. This has been by
far the most productive technique used by planet hunters.
Pulsar timing: A
pulsar (the small, ultradense remnant of a star that has exploded as a supernova)
emits radio waves extremely regularly as it rotates. Slight anomalies in the timing
of its observed radio pulses can be used to track changes in the pulsar's motion
caused by the presence of planets.
Transit method: If a planet crosses (or
transits) in front of its parent star's disk, then the observed visual brightness
of the star drops by a small amount. The amount by which the star dims depends
on its size & on the size of the planet.
Gravitational microlensing: Microlensing
occurs when the gravitational field of a star acts like a lens, magnifying the
light of a distant background star. If the foreground lensing star has a planet,
then that planet's own gravitational field can make a detectable contribution
to the lensing effect.
Circumstellar disks: Disks of space dust surround many
stars, & this dust can be detected because it absorbs ordinary starlight &
re-emits it as infrared radiation. Features in dust disks sometimes suggest the
presence of full-sized planets.
In the future, several space missions are
planned that will employ already proven planet-detection methods. Astronomical
measurements done from space can be more sensitive than measurements done from
the ground, since the distorting effect of the Earth's atmosphere is removed,
& the instruments can view in infrared wavelengths that do not penetrate the
atmosphere. Some of these space probes should be capable of detecting planets
similar to our own Earth. Huge proposed ground telescopes may also be able to
directly image extrasolar planets.
Nomenclature
A lower-case letter
is placed after the star name, starting with "b" for the first planet
found in the system (for example, 51 Pegasi b). The next planet is labeled, for
example, as "51 Pegasi c", the one following that "51 Pegasi d",
& so on. (The letter "a" is not used because it might be interpreted
as referring to the star itself.)
Note that the letters assigned are based on the order in which the planets are discovered, & not on their position. For example, in the Gliese 876 system, the most recently discovered planet is referred to as Gliese 876 d, despite the fact that it is closer to the star than Gliese 876 b or Gliese 876 c.
Before the discovery of 51 Pegasi b in 1995, extrasolar planets were named differently. The first extrasolar planets found around pulsar PSR 1257+12 were named with capital letters: PSR 1257+12 B & PSR 1257+12 C. When a new, closer-in exoplanet was found around the pulsar, it was named PSR 1257+12 A, not D.
Several extrasolar planets have unofficial nicknames, as well. For example, HD 209458 b is unofficially called "Osiris", & 51 Pegasi b is called "Bellerophon". The IAU currently has no plans to officially name extrasolar planets, considering it impractical.
General
properties of exoplanets
All extrasolar planets discovered by radial velocity
(blue dots), transit (red) & microlensing (yellow) to 31 August 2004. Also
shows detection limits of forthcoming space- & ground-based instruments.Most
known exoplanets orbit stars roughly similar to our own Sun, that is, main-sequence
stars of spectral categories F, G, or K. One reason is simply that planet search
programs have tended to concentrate on such stars. But even after taking this
into account, statistical analysis suggests that lower-mass stars (red dwarfs,
of spectral category M) are either less likely to have planets or have planets
that are themselves of lower mass & hence harder to detect. Recent observations
by the Spitzer Space Telescope indicate that stars of spectral category O, which
are much hotter than our Sun, produce a photo-evaporation effect that inhibits
planetary formation.
Stars are composed mainly of the light elements hydrogen & helium. They also contain a small fraction of heavier elements such as iron, & this fraction is referred to as a star's metallicity. Stars of higher metallicity are much more likely to have planets, & the planets they have tend to be more massive than those of lower-metallicity stars.
The vast majority of exoplanets found so far have high masses. All but two of them have more than ten times the mass of Earth. Many are considerably more massive than Jupiter, our own Solar System's largest planet. However, these high masses are in large part an observational selection effect: all detection methods are much more likely to discover massive planets. This bias makes statistical analysis difficult, but it appears that lower-mass planets are actually more common than higher-mass ones, at least within a broad mass range that includes all giant planets. In addition, the fact that astronomers have found several planets only a few times more massive than Earth, despite the great difficulty of detecting them, indicates that such planets are fairly common.
It is believed that the vast majority of known exoplanets are in substantial part gaseous, like the giant planets of our own Solar System. That has only been confirmed, however, for the exoplanets that have been studied with the transit method. A few of the smallest known exoplanets are suspected to be rocky, like the Earth & the other inner planets of our Solar System.
Large planets form soon after star formation. A gas giant such as Saturn or Jupiter typically takes 3 to 30 million years to become a fully fledged planet. Small planets such as the Earth take hundreds of millions of years to form.
Many exoplanets orbit much closer around their parent star than any planet in our own Solar System orbits around the Sun. Again, that is mainly an observational selection effect. The radial-velocity method is most sensitive to planets with such small orbits. Astronomers were initially very surprised by these "hot Jupiters," but it is now clear that most exoplanets (or at least, most high-mass exoplanets) have much larger orbits. It appears plausible that in most exoplanetary systems, there are one or two giant planets with orbits comparable in size to those of Jupiter & Saturn in our own Solar System.
This planetary habitability
chart shows where life might exist on extrasolar planets based on our own Solar
System & life on Earth.The eccentricity of an orbit is a measure of how elliptical
(elongated) it is. Most known exoplanets have quite eccentric orbits. This is
not an observational selection effect, since a planet can be detected about a
star equally well regardless of the eccentricity of its orbit. The prevalence
of elliptical orbits is a major puzzle, since current theories of planetary formation
strongly suggest planets should form with circular (that is, non-eccentric) orbits.
One possible theory is that small companions such as T dwarfs (methane-bearing
brown dwarfs) can hide in such solar systems & can cause the orbits of planets
to be extreme. This is also an indication that our own Solar System may be unusual,
since all of its planets do follow basically circular orbits.
Many unanswered questions remain about the properties of exoplanets, such as the details of their composition & the likelihood of possessing moons. The recent discovery that several surveyed exoplanets lacked water showed that there is still much more to be learned about the properties of exoplanets.[20] Another question is whether they might support life. Several planets do have orbits in their parent star's habitable zone, where it should be possible for Earth-like conditions to prevail. All of those planets are giant planets more similar to Jupiter than to Earth; if these planets have large moons, the moons might be a more plausible abode of life. Detection of life (other than an advanced civilization) at interstellar distances, however, is a tremendously challenging technical task that will not be feasible for many years, even if such life is commonplace.
Notable
extrasolar planets
The first milestone in the discovery of extrasolar planets
was in 1992, when Wolszczan & Frail published results in the journal Nature
indicating that pulsar planets existed around PSR B1257+12.[11] Wolszczan had
discovered the millisecond pulsar in question in 1990 at the Arecibo radio observatory.
These were the first exoplanets ever verified, & they are still considered
highly unusual in that they orbit a pulsar.
The first verified discovery of an exoplanet (51 Pegasi b) orbiting a main sequence star (51 Pegasi) was announced by Michel Mayor & Didier Queloz in Nature on October 6, 1995. Astronomers were initially surprised by this "hot Jupiter" but soon set out to find other similar planets with great success.
Since that time, other notable discoveries have included:
1999, HD 209458 b
This exoplanet, originally
discovered with the radial-velocity method, became the first exoplanet to be seen
transiting its parent star. The transit detection conclusively showed that the
radial velocity measurements suspected to be planets actually were planets.
2001,
HD 209458 b
Astronomers using the Hubble Space Telescope announced that they
had detected the atmosphere of HD 209458 b. They found the spectroscopic signature
of sodium in the atmosphere, but at a smaller intensity than expected, suggesting
that high clouds obscure the lower atmospheric layers.
2003, PSR B1620-26c
On July 10, using information obtained from the Hubble Space Telescope, a
team of scientists led by Steinn Sigurdsson confirmed the oldest extrasolar planet
yet. The planet is located in the globular star cluster M4, about 5,600 light
years from Earth in the constellation Scorpius. This is the only planet known
to orbit around a stellar binary; one of the stars in the binary is a pulsar &
the other is a white dwarf. The planet has a mass twice that of Jupiter, &
is estimated to be 13 billion years old.
2004, Mu Arae d & TrES-1
In
August, a planet orbiting Mu Arae with a mass of approximately 14 times that of
the Earth was discovered with the European Southern Observatory's HARPS spectrograph.
It is the third lightest extrasolar planet orbiting a main sequence star to be
discovered to date, & could be the first terrestrial planet around a main
sequence star found outside the solar system. Furthermore, a planet was discovered
using the transit method with the smallest aperture telescope to date of four
inches. The planet was discovered by the TrES survey, & provisionally named
TrES-1, orbits the star GSC 02652-01324. The finding was confirmed by the Keck
Observatory, where planetary specifics were uncovered.
2005, Gliese 876 d
In June, a third planet orbiting the red dwarf star Gliese 876 was announced.
With a mass estimated at 7.5 times that of Earth, it is currently the second-lightest
known exoplanet that orbits an ordinary main-sequence star. It must almost certainly
be rocky in composition. It orbits at 0.021 AU with a period of 1.94 days.
2005,
HD 149026 b
In July, a planet with the largest core ever discovered was announced.
The planet, HD 149026 b, orbits the star HD 149026, & has a core that is estimated
to be 70 Earth masses, accounting for two-thirds of the planet's mass.
2005,
HD 188753 Ab
In July, the astronomer Maciej Konacki claimed to have discovered
of a roughly Jupiter-mass planet in a relatively tight triple star system. The
stellar trio is about 149 light years away from Earth. This discovery was thought
to raise a challenge to theories about planetary formation, since the presence
of so many stars so close together would probably have disrupted the sort of protoplanetary
disk that is believed to give rise to giant planets. However, in 2007 a team of
astronomers strongly challenged Konacki's conclusion, saying that they saw no
sign of the planet even though their instruments should have been easily capable
of detecting it. Konacki has defended his work & intends to carry out further
observations.
2006, OGLE-2005-BLG-390Lb
On January 25, the discovery of
OGLE-2005-BLG-390Lb was announced. This is the most distant & probably the
coldest exoplanet found to date. It is believed that it orbits a red dwarf star
around 21,500 light years from Earth, towards the center of the Milky Way galaxy.
It was discovered using gravitational microlensing, & is estimated to have
a mass of 5.5 times that of Earth, making it the least massive known exoplanet
to orbit an ordinary main-sequence star. Prior to this discovery, the few known
exoplanets with comparably low masses had only been discovered on orbits very
close to their parent stars, but this planet is estimated to have a relatively
wide separation of 2.6 AU from its parent star.
2006, HAT-P-1b
Using a
network of small automated telescopes known as HAT, Smithsonian astronomers discovered
a planet, since designated HAT-P-1b, that orbits one member of a pair of distant
stars 450 light-years away in the constellation Lacerta. The planet has a radius
1.38 times that of Jupiter, but one-half the mass, making it the least dense planet
on record (about one quarter that of water). It remains unclear how such a planet
could evolve, & it is believed this object & HD 209458 b (also a low-density
giant planet) could ultimately provide insight on how planets form. According
to Robert Noyes of the Harvard-Smithsonian Center for Astrophysics (CfA), "We
can't dismiss HD 209458 b as a fluke. This new discovery suggests something could
be missing in our theories of how planets form."
2006, SWEEPS-10
The
planet candidate with the shortest orbital period yet found, named SWEEPS-10 (SWEEPS
stands for Sagittarius Window Eclipsing Extrasolar Planet Search), completes an
orbit of its star in just 10 hours. Located only 1.2 million kilometers from its
star (roughly three times the distance between the Earth & the Moon), the
planet is among the hottest ever detected; its estimated temperature is approximately
1650 degrees Celsius. "This star-hugging planet must be at least 1.6 times
the mass of Jupiter, otherwise the star's gravitational muscle would pull the
planet apart," said team leader Kailash Sahu of the Space Telescope Science
Institute in Baltimore, Maryland. Such ultra-short period planets (USPPs) seem
to occur only around dwarf stars. The small star's relatively low temperature
allows the planet to exist. "USPPs occur preferentially around normal red
dwarf stars that are smaller & cooler than our Sun," Sahu said.
2007,
HD 209458 b & HD 189733b
On February 21, 2007, NASA & Nature released
news that HD 209458 b & HD 189733 b were the first two extrasolar planets
to have their spectra directly observed. This was long seen as the first mechanism
by which extrasolar but non-sentient life forms could be searched for, by way
of influence on a planet's atmosphere. A group of investigators led by Dr. Jeremy
Richardson of NASA's Goddard Space Flight Center were first to publication, in
the February 22 issue of Nature. Richardson et al. spectrally measured HD 209458
b's atmosphere in the range of 7.5 to 13.2 microns. The results defied theoretical
expectations in several ways. The spectrum had been predicted to have a peak at
10 microns which would have indicated water vapor in the atmosphere, but such
a peak was absent, indicating no detectable water vapor. Another, unpredicted
peak was observed at 9.65 microns, which the investigators attributed to clouds
of silicate dust, a phenomenon not previously observed. Another unpredicted peak
occurred at 7.78 microns, which the investigators did not have an explanation
for. A separate team led by Mark Swain of the Jet Propulsion Laboratory also observed
HD 209458 b's spectrum, & indicated that their findings were similar. They
had submitted their results to Astrophysical Journal Letters. A team led by Carl
Grillmair of NASA's Spitzer Science Center made the observations of HD 189733
b, & their results were pending publication in Astrophysical Journal Letters
at the time of the news release.
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An Index with links to almost all our sites.
A Picture of Jupiter, & a Picture of Florida from Space
A Top picture of a Space Shuttle taking off A Top picture of Stars A Nebula
SOLAR SYSTEM RECORD BREAKERS, facts like the tallest mountain for the planets
A site saying the 10 most famous aliens ever
A Clock saying how many people have landed on each planet in our Solar System
Jokes to say to aliens that are offending you
Joke names for aliens, our worst Space site
A List of space associated TV programmes & Movies
http://www.lonympics.co.uk/Olympus Mons.htm Tallest mountain, & volcano in the solar system, & Mars.
A Picture of Buzz Aldrin on the Moon
What would happen if Marsians invaded
A Multiple Choice Quiz on Space
The Moon Deimos The Moon Phobos
Some interesting facts about Space
How Many people have landed on each of our Planets see, in this space clock
A list of Planets that could be colonised by humanity
Some plans for colonising Venus
Jupiter
- the facts
The Moon - the facts
A Quiz on space related TV & movies
The
facts on Space Stations Jodrell
Bank the facts
Ganymede
the facts The Facts on the
Moon Europa
Some ideas for stories, & movie ideas I have had, including some on space
One
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same site in Spanish
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Global Geography, sites like what are the 10 largest English speaking countries, 10 largest Celtic cities, biggest forests, volcanoes,
SOLAR SYSTEM RECORD BREAKERS, facts like the tallest mountain for the planets
Which are the 10 most powerful countries in 2008___
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100s of fantastic websites http://www.lonympics.co.uk/
Car insurance quotation - but with lots of famous quotes on the issue of cars
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