Extrasolar planet -
Cont'd
Solar system formation processes
One question raised by the detection of extrasolar planets is why
so many of the detected planets are gas giants which, in comparison
to Earth's solar system, are unexpectedly close to the orbited
star. For example, Tau Boötis has a planet 4.1 times Jupiter's
mass, which is less than a quarter of an astronomical unit (AU)
from the orbited star, that is closer to the star than Mercury
orbits the sun. HD 114762 has a planet 11 times Jupiter's mass,
which is less than half an AU from the orbited star. One possible
answer to these unexpected planetary orbits is that since
astrometrics detects the extrasolar planets due to their
gravitational influences and partially-ecliptic interference,
perhaps current technology only permits the detection of systems
where a large planet is close to the orbited star, rather than such
systems being the norm.
The frequency of extrasolar planets is one of the parameters in the
Drake equation, which attempts to estimate the probability of
communications with extraterrestrial intelligence.
Notable extrasolar planets
- In 1992, Wolszczan and Frail published results indicating that
pulsar planets existed around PSR B1257+12 in Nature, volume 355,
145-147. Wolszczan had discovered the millisecond pulsar in
question in 1990 at the Arecibo radio observatory. These were the
first exoplanets ever verified, all the much more rare, that they
orbit a pulsar.
- The first verified discovery of an exoplanet (51 Pegasi B)
orbiting a main sequence star (51 Pegasi) was announced on October
6, 1995 by Michel Mayor and Didier Queloz in Nature, volume 378,
page 355.
- A microlensing event in 1996 of the gravitationally lensed quasar
Q0957+561, observed by R. E. Schild in the A lobe of the double
imaged quasar, has led to a controversial, and unconfirmable
speculation that a 3 Earth mass planet is possibly in the unknown
lensing galaxy, between Earth and the quasar. This would be the
most distant planet, if it could be confirmed, and is assumed to
reside at redshift 0.39; 2.4 Gpc away (7.8 billion light years or
74 Ym), where the lensing galaxy is. The double-image quasar
itself, called The Twin Quasar or Old Faithful, Q0957+561 A/B,
resides at redshift z=1.41.
- In 1999, HD 209458b was the first exoplanet seen transitting its
parent star, conclusively proving that the radial velocity
measurements that were planets actually were planets.
- On November 27, 2001, astronomers using the Hubble Space
Telescope announced that they had detected the atmosphere of the
planet orbiting HD 209458 (known as HD 209458b and provisionally
dubbed "Osiris"). Also during that year, a star was located which
had the remnants of one or more planets within the stellar
atmosphere apparently the planet was mostly vaporized. It has
been suggested that there may be planets that orbit so closely to
their suns that most of their mass has been stripped away by the
heat, provisionally referred to as Chthonian planets.
- On July 10, 2003, using information obtained from the Hubble
Space Telescope, scientists discovered the oldest extrasolar planet
yet. Dubbed Methuselah after the biblical figure, the planet is
about 5,600 light years from Earth, has a mass twice that of
Jupiter, and is estimated to be 13 billion years old. It is located
in the globular star cluster M4, in the constellation
Scorpius.
- On April 15, 2004, separate teams announced the discoveries of
three planets outside our solar system.
-- One of which is 17,000 light years away, more than three times
farther away than the previous record holder. The background star
that was used in the gravitational lensing is 24,000 light-years
away. The newly-discovered exoplanet is estimated to be about 1.5
times the mass of Jupiter and presumed to be similarly gaseous. It
orbits the star about 3 astronomical units (AU). Jupiter is 5.2 AU
from the Sun.
-- The same day, a European team of planet hunters based at the
Geneva Observatory found two giant planets using the transit
method. Both planets are called "hot Jupiters," close to one
Jupiter-mass but orbiting its star so closely that it completes an
orbit in less than two earth days.
- In August 2004, a planet orbiting mu Arae with a mass of
approximately 14 times that of the Earth (see link) was discovered with the ESO HARPS
spectrograph. It is the lightest extrasolar planet orbiting a main
sequence star to be discovered to date, and could be the first
terrestrial planet around a main sequence star found outside the
solar system.
- In August 2004, a planet was discovered using the transit method
with the smallest aperture telescope to date, 4 inches. The planet
was discovered by the TrES survey, and provisionally named TrES-1,
orbits the star GSC 02652-01324. The finding was confirmed by the
Keck Observatory, where planetary specifics were uncovered.
Candidate planets
- On September 10, 2004, a team of European and U.S. astronomers
announced (see link) what may be the first direct optical
observation of an extrasolar planet, orbiting the brown dwarf
2M1207, 230 light-years from Earth. Its spectrum, which shows the
presence of water, and other characteristics make the object almost
certainly a planet, but further observations will be required to
confirm that it does in fact orbit the star next to which it was
photographed. The planet is believed to be a young gas giant
approximately 5 times as massive as Jupiter, and to have an orbital
radius of approximately 55 AU, which would also make it farther
from its host star than any exoplanet previously detected by a
factor of nine.
- In April 2005, a team of European astronomers released the first
direct image of an extrasolar planet, GQ Lupi b, orbiting the main
sequence star GQ Lupi, 400 light-years from Earth. Its orbital
radius is approximately 100 AU.
Issue #46
