PLANETS AROUND OTHER STARS

``There is not merely one world, one earth, one sun, but as many worlds as we see bright lights around us.''
- Giordano Bruno

Key Concepts

(1) Planets around other stars can be detected using the Doppler effect.

The Sun is not an extravagantly unusual star. The Galaxy contains brighter stars and fainter stars, bigger stars and smaller stars, hotter stars and colder stars, younger stars and older stars. If the Sun is an ordinary, everyday sort of star, then we might expect other ordinary stars to be accompanied by planetary systems as well.

Before the time of Copernicus, it was thought that the Sun was a very different object from the stars. The Sun, in the geocentric model, was a big glowing sphere, and the stars were mere points of light attached to the celestial sphere. In the Copernican model, however, this viewpoint changed. The stars, Copernicus realized, had to be at a great distance from Earth. This raised the possibility that the stars were big glowing spheres just like the Sun, and that they appeared as dim points of light simply because of their great distance.

Giordano Bruno (1548 - 1600), an Italian philosopher, was an early adherent to the Copernican model. Bruno believed that the universe was infinite, containing an infinite number of stars similar to the Sun. These stars, Bruno believed, were surrounded by planets, and the planets were inhabited by intelligent life. Bruno's ``many worlds'' hypothesis was deemed heretical by the Church. If intelligent beings existed elsewhere, did they have souls? Did they suffer from original sin? Did they have their own messiah? The whole concept was worrisome.

Unfortunately for Bruno, his belief in planets orbiting other stars was just a minor detail in his very original, very clever, and very heretical theology. Bruno equated the infinite universe with an infinite God, in a version of pantheism that was anathema to all Christians. Bruno was eventually convicted of heresy by the Inquisition and was burned at the stake in Rome. Although Bruno was not primarily condemned for his astronomical beliefs, his death nevertheless had a dampening effect on speculations about planets and life beyond the Solar System.

Detecting extrasolar planets is difficult (but not impossible!) To illustrate the difficulty, imagine you are an astronomer on a planet orbiting Proxima Centauri, 4.2 light years (about 270,000 A.U) away from the Sun. You want to determine whether any planets are orbiting the Sun. The easiest of the Sun's planets to detect is Jupiter, the biggest planet, which is 5.2 A.U away from the Sun. As seen from Proxima Centauri, Jupiter is one billion times fainter than the Sun, and is only 4 arcseconds away from the Sun. At such a small distance from the Sun, it will be lost in the Sun's glare. (Note that Proxima Centauri is the Sun's nearest neighbor. As seen from a more distant star, the angular distance between the Sun and Jupiter will be even smaller, and Jupiter will be even harder to detect.)

Since seeing planets directly is so difficult, it is time to be indirect and devious. It is easier to detect Jupiter from a distance by detecting its gravitational influence on the Sun. Recall from our discussion of orbits that (strictly speaking) Jupiter doesn't orbit the Sun. Rather, Jupiter and the Sun both orbit the center of mass of the Jupiter - Sun system. Since the Sun's mass is roughly a thousand times the mass of Jupiter. Thus, the center of mass of the Jupiter - Sun system is displaced from the Sun's center by a distance equal to roughly 1/1000 the Sun - Jupiter distance. Since the distance from the Sun's center to Jupiter's center is 5.2 A.U., on average, the center of mass of the Sun-Jupiter system is 0.0052 A.U., or 780,000 kilometers, from the Sun's center. Since the radius of the Sun is only 696,000 kilometers, this means that the Sun, thanks to Jupiter's gravitational influence, is orbiting a point slightly above its surface.

As Jupiter travels along its orbit, with a semimajor axis of 780 million kilometers, the Sun travels along a smaller orbit with a semimajor axis of 780 thousand kilometers. The orbital periods of Jupiter and the Sun are the same. Thus, while Jupiter goes zipping along with an average speed of 13 kilometers/second, the Sun dawdles along with an average speed of 13 meters/second (about 29 mph).

Thus, astronomers at Proxima Centauri (or anywhere else) can indirectly deduce the presence of Jupiter by looking for variations in the Doppler shift of light from the Sun. The Sun will be slightly blueshifted when its little orbit carries it toward Proxima Centauri, and slightly redshifted when its orbit carries it away from Proxima Centauri.

A technique that allows astronomers from Proxima Centauri (or any other star) to detect planets orbiting the Sun will allow astronomers from Earth to detect planets orbiting Proxima Centauri (or any other star). There are a few technical difficulties, of course. A speed of 13 m/sec is only 43 billionths the speed of light, and thus will produce a shift in wavelength of only 43 parts per billion. Detecting such a tiny redshift or blueshift requires sophisticated technology (which is why this technique for detecting planets has only recently been successful).

(2) Planets have been detected around stars other than the Sun.

The ``Doppler shift'' technique outlined above was first used to detect a planet orbiting the star 51 Pegasi, a star in the constellation Pegasus which is very similar in temperature and luminosity to our own Sun. 51 Pegasi has a variation in its observed velocity with an amplitude of 56 meters/second and a period of 4.2 days. This implies that 51 Pegasi is orbiting with a maximum speed of 56 meters/second and an orbital period of only 4.2 days. The deduced properties of the planet perturbing 51 Pegasi: it is half as massive as Jupiter and is only 0.05 A.U. from the star. (For comparison, Mercury is 0.39 A.U. from the Sun).

There are a few caveats attached to the Doppler shift technique for detecting planets. First, the technique only works if the orbital velocity of the star is large enough to be detected. Currently, velocities smaller than 3 meters/second are too tiny to be observed. In the solar system, for instance, only Jupiter produces a effect large enough to be measured by this technique. Saturn perturbs the Sun's velocity by only 2.8 meters/second, and the perturbations due to the other planets are still smaller. The Doppler technique only discovers

A second caveat is that the masses deduced for the planets are actually lower limits. The deduced masses assume that we are looking at the orbit edge-on. This is merely an assumption, since we can't see the orbit directly. If we are viewing the orbit at an angle, the radial velocity that we detect from the Doppler shift is only part of the star's total orbital velocity. A bigger total orbital velocity then implies that a more massive planet is tugging the star around.

As of October 31, 2003, the Extrasolar Planets Encyclopedia web site lists 102 planetary systems with 117 confirmed planets around stars other than the Sun. This number is steadily growing, however. A very preliminary guess is that about 5 percent of Sun-like stars have planets comparable in mass to Jupiter within 5 A.U. of the central star.

The most interesting planetary system found to date is around the star Upsilon Andromedae. This star is 44 light years away from the Sun in the constellation Andromeda; it is slightly hotter and more massive than the Sun. The radial velocity of Upsilon Andromedae has been monitored for several years, and shows a complicated wobble that cannot be accounted for by the presence of a single planet. The best fit to the wobble is given by a model with THREE planets orbiting Upsilon Andromedae:

A comparison of the Upsilon Andromedae planetary system with the inner Solar System is shown below.


[Image credit: Harvard-Smithsonian Center for Astrophysics]

(3) Some planetary systems have very different properties from our Solar System.

Astronomers have been greatly astonished by these newly discovered planets around other stars. The major surprise was that many times we see high-mass planets (presumably similar in structure to the Jovian planets in our own solar system) extraordinarily close to the star which is it orbiting. Of the first 21 stars discovered to have planets orbiting them, 16 have Jupiter-like planets on orbits smaller than that of Mercury. Admittedly, the search technique is biased toward finding large planets with high-speed orbits; still, the standard nebular theory for the formation of the solar system states that Jovian planets can't form close to a star - it's just too hot there.

What gives? Why are these massive planets so very close to their parent star? At present, most astronomers favor the hypothesis that massive Jovian planets can form far from a central star, then spiral inward due to forces in the early solar nebula. This particular hypothesis raises a few questions...Why don't the Jovian planets spiral all the way in, and be swallowed up by the central star? Why haven't Jupiter, Saturn, Uranus, and Neptune in our own solar system spiraled inward? Obviously, much work must still be done on the problem. Astronomers are still confused. (Actually, astronomers are often confused. Science is never as neat as it appears. Hypotheses must continuously be tinkered with, as we learn more and more about the universe around us.)

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