Are there planets orbiting other stars beyond our solar
system? We do not know for sure, but with the recent discoveries
about 51 Pegasi, 70 Virginis and 47 Ursae Majoris the weight of
evidence is now so strong that only a "devil's
advocate" denies the conclusions. Here is some of what we do
know (this is somewhat incomplete; please see the references
below for more info):
- Three small bodies have been found in orbit around the
pulsar PSR 1257+12. They have been designated
"PSR1257+12 A, ..B, and ..C". One is about the
size of the Moon, the other two
are about 2 to 3 times the mass of Earth.
They were discovered by measuring variations in the
pulsation speed of the pulsar which can be interpreted as
gravitational effects of three small planets. The
original observation has been confirmed but, of course,
no direct images have been made -- that is way beyond the
capabilities of our best telescopes.
These planets are believed to have formed after the
supernova that produced the pulsar. The present planets
would have originally been within the envelope of the
progenitor star and therefore wouldn't have stood much
chance of surviving the supernova explosion, and wouldn't
have remained in circular orbits after the explosion.
Several decades of timing data on the pulsar PSR
0329+54 (PKS B0329+54) by Tatiana Shabanova (Lebedev
Physics Institute) shows evidence of a planet with a 16.9
year period and mass >= 2 Earth masses.
But, while the evidence for these is pretty good, they
aren't really what we're looking for when we talk about
'solar systems'.
- It has been known since 1983 that the star Beta
Pictoris is surrounded by a disk of gas and dust.
Spectra of Beta Pictoris show absorption features which
are currently believed to be due to cometary like clouds
of gas occultating the star from the debris left over
from planetary formation. Though it's far from certain it
is believed by some that planets may already have formed
around Beta Pictoris.
HST
has 
observed Beta
Pictoris (right) and found the disk to be significantly
thinner than previously thought. Estimates based on the
Hubble image place the disk's thickness as no more than
one billion miles (1600 million kilometers), or about 1/4
previous estimates from ground-based observations. The
disk is tilted nearly edge-on to Earth. Because the dust
has had enough time to settle into a flat plane, the disk
may be older than some previous estimates. A thin disk
also increases the probability that comet-sized or larger
bodies have formed through accretion in the disk. Both
conditions are believed to be characteristic of a
hypothesized circumstellar disk around our own Sun, which
was a necessary precursor to the planet-building phase of
our Solar Systems, according to current theory.
More recent HST observations have shown the disk to be
slightly warped as might be expected from the
gravitational influence of a planet. This has been
confirmed by observations
at ESO.
- Recent observations at radio wavelengths of a gas cloud
known as Bok Globule B335 have produced images of
material collapsing onto a newly born star (only about
150,000 years old). These observations are helping to
understand how stars and planets form. The phenomena
observed matches the theory of the formation of the solar
system -- that is, a large gas cloud collapsed to form a
star with an attendant circumstellar disk in which, over
time, planets accreted from the matter in the disk and
orbited the Sun.
- The IRAS satellite found that Vega had too much infrared
emission, and that has been attributed to a dust shell
(with a mass of maybe Earth's moon).
- Observations of the very nearby Barnard's Star were once
thought to be evidence of gravitational effects of
planets but they now seem to have been in error.
- The star Gl229 seems to contain a 20 Jupiter mass object
orbiting at a distance of 44 AU. An object this large is
probably a brown-dwarf rather than an ordinary planet.
- What may be the first discovery of a planet orbiting a
normal, Sun-like star other than our own has been
announced by astronomers studying 51 Pegasi, a
spectral type G2-3 V main-sequence star 42 light-years
from Earth. At a recent conference in Florence, Italy,
Michel Mayor and Didier Queloz of Geneva Observatory
explained that they observed 51 Pegasi with a
high-resolution spectrograph and found that the star's
line-of-sight velocity changes by some 70 meters per
second every 4.2 days. If this is due to orbital motion,
these numbers suggest that a planet lies only 7 million
kilometers from 51 Pegasi -- much closer than Mercury is to the Sun -- and that
the planet has a mass at least half that of Jupiter. These physical
characteristics hinge on the assumption that our line of
sight is near the planet's orbital plane. However, other
evidence suggests that this is a good bet. A world merely
7 million km from a star like 51 Pegasi should have a
temperature of about 1,000 degrees Celsius, just short of
red hot. Probably lacking an atmosphere, the planet may
be a nearly molten ball of iron and rock with seven times
the Earth's diameter and seven
times its surface gravity. One side may permanently face
the star, much as the Moon's does
the Earth.
These observations have now been confirmed
by several independent observers. And there is some
evidence for a second planet much farther out that is not
yet confirmed.
[ The 5.5-magnitude 51 Pegasi is easily visible in
binoculars, high in the evening sky, between Alpha and
Beta Pegasi, the western pair of stars in the Great
Square of Pegasus. The star's equinox-2000 coordinates
are R.A. 22 hours 57 minutes, Dec. +20 degrees 46
minutes. ]
- On 1/17/96 Geoffrey
Marcy and Paul Butler
announced the discovery of planets orbiting the stars 70
Virginis and 47 Ursae Majoris. 70 Vir is a G5V
(main sequence) star about 78 light-years from Earth; 47
UMa is a G0V star about 44 light-years away. These were
discovered using the same doppler shift technique that
found the planet orbiting 51 Pegasi.
The planet around
70 Vir orbits the star in an eccentric, elongated orbit
every 116 days and has a mass about nine times that of
Jupiter. Using standard formulas that balance the
sunlight absorbed and the heat radiated, Marcy and Butler
calculated the temperature of the planet at about 85
degrees Celsius (185 degrees Fahrenheit), cool enough to
permit water and complex organic molecules to exist. The
star 70 Vir is nearly identical to the Sun, though
several hundred degrees cooler and perhaps three billion
years older.
The planet around 47 UMa was discovered after analysis
of eight years of observations at Lick Observatory. Its
period is a little over three years (1100 days), its mass
about three times that of Jupiter, and its orbital radius
about twice the Earth's distance from the Sun. This
planet too probably has a region in its atmosphere where
the temperature would allow liquid water.
- As of April 1996, Drs. Marcy and Butler have discovered
yet another planet this time around the star HR3522 (aka Rho
1 Cancri, 55 Cancri) about 45 light years from the
Earth. The planet is estimated to be about 0.8 Jupiter
masses. It is likely that several more planets will show
up in the initial set of 120 stars that they have
monitored.
- Several more extra-solar planets have now been discovered
by the Butler/Marcy method. It seems likely that there
are a very large number of such planets out there.
- Another extra-solar planet has been discovered orbiting
16 Cygni B. But unlike all other previously known planets
this one has a very large orbital eccentricity (0.6); its
orbit carries it from a closest distance of 0.6 AU from
its star to 2.7 AU. This calls into question many
theories of planetary formation.
- Detecting extra-solar planets directly is very difficult.
Even the Hubble Space
Telescope wouldn't be able to image planets at the
expected sizes and distances from their suns.
What HST
did find were disks of matter around stars seen in 
silhouette against
the Orion
Nebula (called 'proplyds', for 'proto-planetary
disks' (right). This is great evidence for how common
these objects are, but the scale is way too small to say
anything directly about planets there. More detailed
HST images are now available, too.
- Nevertheless, it might be possible to detect the
infra-red radiation of very large planets (Jupiter-sized
or more) in some circumstances.
- By a stroke of good luck, HST has taken an image of what
appears to be a planet escaping from a double star
system. See the 1998
May 28 announcement. If this is confirmed, the
existence of extrasolar planets will be undeniable.
silhouette against
the Orion
Nebula (called 'proplyds', for 'proto-planetary
disks' (right). This is great evidence for how common
these objects are, but the scale is way too small to say
anything directly about planets there. More detailed
HST images are now available, too.