Other Planets Orbiting Other Stars
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
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 greater than 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
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
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
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. It
was initially thought that it might be a solid body like a very big Mercury
but the concensus now seems to be that it is a "hot Jupiter", a gas
planet formed much farther from its star that migrated inward.
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
[ The 5.5-magnitude 51 Pegasi is easily visible in binoculars 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 andPaul 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
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.
| Home | Course
Assignments | Teacher Bio
| Course Units
| Syllabus | Links