Y a-t-il des planètes en orbite autour d'étoiles au-delà de notre système solaire ?
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):
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Les faits
- 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.
- 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.
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Controverses
- It is likely that more planetary systems will be discovered using the methods
that found 51 Pegasi, 70 Virginis and 47 Ursae Majoris. Exciting times are ahead!
- The 51 Pegasi planetary system is quite different from our solar system.
But the 70 Virginis and 47 Ursae Majoris systems appear to be more "normal".
With 4 known "solar systems" it is now possible to make more general statements
and to better test theories of planetary formation.
- None of the extra-solar planets discovered so far are at all similar to the Earth,
as expected given our current methods. How can we get more evidence about
extra-solar Earth-like planets? Will the ExNPS program get funded?
- How do you get a planet with a large eccentricity?
- [abstract of the above paper by Dr. Fraknoi]:
Interest among astronomers in the detection of extra-solar
planets is accelerating with the growing realization that it may
soon be technically feasible. The ongoing renaissance in
telescope construction and the anticipated launches of new
space platforms are encouraging many scientists to review
and improve the means by which planets can be discovered.
The direct detection of the light from a distant planet would be
the most compelling means of discovery and to gauge the
feasibility of various search strategies, astronomers have
traditionally used the current Jupiter as a benchmark planet.
However, in principle, extra-solar giant planets (EGPs) can
have a wide range of masses and, hence, can be significantly
brighter than Jupiter. Furthermore, the maximum mass a
planet can have is not known a priori, and observations will
be needed to determine it. We predict the optical and infrared
fluxes of EGPs that searches in the next few years may reveal.
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