Then vs. Now: Extra-Solar Planets
An artist's concept of an extra-solar
planet moving behind its parent star. Credit: NASA/JPL-Caltech/R. Hurt (SSC)
Ten years ago the boom in finding planets around other stars was
just starting. We quickly went from a few suspected planets, and
planets around exotic objects, to the discovery of many Jupiter-sized
planets around normal stars. Today there have been nearly 200 planets
discovered around nearby stars,
with the smallest less massive than Uranus. In addition, there are
now prospects for discovering Earth-sized planets.
In a planetary system, the planets and the parent star orbit around
a common center of gravity. (In our solar system, the Sun orbits
around around this common center of gravity with a speed of 12 m/s).
While we cannot see the planets and their motion, we can see the star
move. Astronomers have discovered planets by utilizing various
techniques for detecting the motion of the star.
Before 1995, there had been searches for planets around nearby stars
using a "proper motion" technique. For some nearby stars, we can see
the motion of the star through space. If the star has an a planet,
then the common center of gravity would move in a straight line and
the star would appear to wobble along this straight line path.
Peter van de Kamp (Swarthmore College) examined the motion of Barnard's star
using photographic plates taken from 1938-1981. He claimed to detect
a wobble in the proper motion of Barnard's star, and deduced that there
were two planets in circular orbits: one was 0.7 Jupiter masses (or
"Jupiters") with an orbital period of 12 years, the other 0.5 Jupiters
with period of 20 years. These results however were never
independently confirmed. Van de Kamp died in 1995 standing behind his results.
In the early 1990's planets were also discovered
around pulsars. The precision timing of the pulses from a pulsar
allow an accurate diagnostic of the pulsar's motion. A sinusoidal
variation in the arrival time of the pulse indicates the pulsar's
around an unseen body. In 1992, Aleksander Wolszczan and
Dale Frail announced the
discover of a planetary system around the pulsar PSR
1257+12. Subsequent observations confirmed and refined the parameters
of the system: 3 planets, with masses ranging from 0.015 of the mass
of the Earth to 4.3 times the mass of the Earth. The distances of the
planets from the pulsar range from 0.20 to 0.46 AU. Evidence for a
fourth body was discovered in 2002, but it is thought to be akin to an
asteroid or a comet. To date 4 pulsars have been found to have planets.
Since 1983, the star Beta Pictoris has been known to have a dust ring
around it. A void in the dust ring suggests the presence of a planet,
which sweeps out the dust. Using numerical models, Francois Roques
and D. Lazzaro have suggested that the planet is 20 AU from the star and
moving in a nearly circular orbit. The mass of the planet is about 5
times the mass of the Earth. While observations are consistent
with this model, the planet within the dust ring has not been
The Planet Boom
In 1995, Michel Mayor and Didier Queloz announced their discovery of a planet orbiting the
star 51 Peg. This was soon confirmed by a number of groups. The
planet has mass of about half of Jupiter, lies at a distance of 0.053
AU from the star, and has a circular orbit with a period of 4.2 days.
A planet this massive and this close to its parent star was certainly
not what everyone was expecting!
But the technique used to detect this planet is particularly
sensitive to this type of planet. It was discovered by measuring the
radial velocity of the star over a long period of time. In this
technique, astronomers measure the Doppler shift of the spectral
lines from the star.
Observing over the course of months or years, the data show a periodic shift
in this Doppler velocity as the star moves toward us and away from
us. From this we detect the orbital
motion of the star around the center of gravity, and can deduce the
mass(es) and distance of planet(s) in the system. Capable of measuring
velocities down to 1 m/s, this technique favors large planets and/or
planets near their parent star. This technique is effective for
stars within about 150 light years from Earth.
In January 1996, Geoffrey Marcy and Paul Butler announced the discovery of
planets around 70 Virginis (planet mass = 7.5 Jupiter, oribital period = 116.7
d, distance from star = 0.48 AU) and
47 Ursa Majoris (planet mass = 3 Jupiter, orbital period = 1100 days,
distance from star = 2 AU). Marcy and Butler had been
monitoring 100 nearby stars over the previous 8 years.
In April 1996, Marcy and Butler announced the presence of a planet around HR3522 (= 55
Cancri = Rho 1 Cancri). The planet mass is 0.8 Jupiter, with a period
of 14.6 days, and a distance from the star of 0.114 AU. Later,
3 more planets were discovered around this star.
But the first multiple planet system was announced in 1999, when 3
planets were discovered around Upsilon Andromedae. The planet masses
range from 0.7 to 3.95 Jupiters, and their distances from the parent
star range from 0.06 to 2.54 AU.
As time has passed, more planets have been discovered by a variety
of groups which had monitored a number of stars over a number of years.
Since 1999, between 12 and 26 new planets have been found each year!
The radial velocity curve for the
planet discovered around HD 11964 in June 2006. The dotted line is the
best fit curve through the date points. It indicates a minimum mass
(given by M sin i) of 0.6 Jupiter, and an orbital period of 5.8 years
Butler et al, 2006).
By mid-2006, nearly 200 planets had been discovered around 160
main sequence stars. Twenty of these stars have multiple planet
The parent stars have masses ranging from 0.3 - 2.0 solar masses, with
distances from Earth ranging from 3.2 - 100 parsecs.
The masses of the planets range from 0.0185 Jupiter (= 5.9 times
the mass of the Earth) to 23 Jupiters. The orbital periods range from
1.33 days to a
few exceptional cases of 25 years. The distances from parent star
range from 0.018 - 9.21 AU (for comparison, Mercury orbits at
0.39 AU from the Sun, and Saturn orbits at 9.54 AU). The orbital
eccentricities range from 0.0 (circular) to 0.94.
masses of exoplanets
orbital periods of exoplanets
orbital distances of exoplanets|
Graphs showing the distribution of exoplanet
masses, periods, and distances from parent star. Planets with
distances > 3 AU have periods longer than most surveys, and thus are
underrepresented. (Click on graphs for larger images.) Credit:
Butler et al, 2006)
Other Searches, Other Techniques
The future of planet searches lies with finding planets of smaller
sizes, down to the size of the Earth.
If circumstances are fortuitous, scientists can observe a planet
crossing the disk of its parent star. In this "transit" technique, a
small amount of light from the star is blocked by the planet, and
astronomers observe a slight decrease in the star's intensity.
By studying this dimming, astronomers can deduce the size of the
planet, its density, and whether it has an atmosphere. Nine
extrasolar planets have been studied in this manner.
The infrared light curves of the parent
stars for the planets TrES-1 (top) and HD 209458b (bottom). The plots
show the decrease in infrared light when the planet passes behind its
parent star. The decrease can be used to infer properties of the
(Credit: NASA/JPL-Caltech/D. Charbonneau (Harvard-Smithsonian CfA)
and NASA/JPL-Caltech/D. Deming (NASA/GSFC)
Jeffrey Newman (UC Berkeley) and NASA)
Likewise, light from the planet itself can be studied when
the planet passes behind its parent star. Astronomers using the
Spitzer Space Telescope first used this trick on the planets HD209458b
and TrES-1. First, the total infrared light of the parent star and
planet is measured. Then, when the planet passes behind the star,
astronomers can measure the infrared light of just the star. The
difference between these two observations gives the infrared light of
the planet, from which the planet's temperature can be obtained.
For HD209458b, astronomers found a temperature of 1130 K, and for
TrES-1 a temperature of 1060 K.
Planets via Gravitation Microlensing
Four planets have been discovered via gravitational microlensing.
Gravitational microlensing occurs when light from a distant star
passes by a star that is closer to us. The gravitational bending of
the light also changes the intensity of the distant star light. Since
the stars are in motion, the intensity varies as the closer star
passes in front of the distant one. If
the foreground star has a planetary companion, the planet contributes
to the lensing event, and its presence can further alter the intensity.
In two star systems, planets with Jupiter-like mass a few AU from the
parent star were discovered. But in 2005 and 2006, planets with
masses 5.5 Earths and 13 Earths were discovered, each around 3 AU from
their parent star. Thus, this technique has the ability to detect
Earth-mass planets using Earth-bound observatories.
Publication Date: August 2006