4. EXTRA-SOLAR PLANETS AND BROWN DWARF STARS

Be sure to read this section carefully. This is one of the hottest subjects in astronomy today, and it is not mentioned in your textbook. You can also find excellent summaries of this topic in this article from Scientific American.

The formation of planetary systems: the current theory for the origin of the solar system and other planetary systems has its origins in early (1755) ideas by the philosopher Immanuel Kant. The basic idea is that planets will form out of the protoplanetary disk of gas and dust that swirls around a newborn star. If the disk is dense enough, the dust grains will tend to stick to each other, bonded by ices, and eventually accumulate into planetesimals. (Astronomers are interested in comets because they are thought to be similar to planetesimals. The planetesimals keep growing, eventually becoming massive enough to attract and capture gas from the nearby disk and build up gaseous atmospheres. The giant planets capture the little ones, and sometimes fling them out of the planetary systems. This process is illustrated below.

This process is also illustrated with this movie from CNN.

When astronomers developed this theory mathematically, they found that giant gaseous planets like Jupiter could only form at distances greater than about 5 AU - where Jupiter lies. There were two reasons. The first is that tidal forces would prevent such a planet from forming, and the second is that the light from the star would heat the gas, preventing accumulation. According to this theory, only smaller rocky planets like the Earth, Venus, Mars, Mercury, could form so close to the star. When such planets formed, they would fling out many other planets, leaving just a few behind. But as you will shortly see, when astronomers did discover planets around other stars, they were massive gaseous planets like Jupiter and they were closer to their companion stars than Mercury is to the Sun. So much for that theory!

The discovery of extrasolar planets: Four years ago, we wouldn't have much to say about planets around other stars because none were known. The first planet orbiting a normal star was discovered in 1995 by two Swiss astronomers. Today, astronomers have discovered more than 70 nearby stars like the Sun that have planets orbiting around them. Click here for an up-to-date catalogue of extrasolar planets: Extrasolar Planets Encyclopedia. You will see that astronomers have found more than 70 planets with masses ranging from about 0.16 times Jupiter's mass to 14 times Jupiter.

Where are the stars with planets? They are all relatively near to the Sun, at distances less than 60 parsecs. Of course, there must be many more, but our present technology only allows us to find them within this distance.

Most of the known extrasolar planetary systems have been discovered by two astronomers from California, Goeff Marcy and Paul Butler. You should check Discovery of Extrasolar Planets, which has a very nice illustration comparing these planetary systems to the solar system. If you are enjoying this, you might also want to check Extrasolar Visions, which presents the same information in a very attractive format with some speculative paintings of what the planets might look like. It is striking that most of these systems have massive planets in orbits comparable to or smaller than the orbit of Mercury. In one sense this result is not surprising; the heavier the planet is and the closer it is to its star, the easier it is to discover. With existing techniques, it is still impossible to detect a planet like the Earth around a normal star (except pulsars). Moreover, the larger the planet's orbit, the longer it takes to discover.

But in another sense the discovery was very surprising, because there was a theory that said that such planets couldn't exist. What was wrong with the theory? According to current thinking (which may change) the original theory is still OK: giant planets can't form very close to the star. But astronomers have added a new idea that seems to make sense: the giant planets will form further out, like Jupiter, but can spiral in toward the central star while there is still enough material in the protoplanetary disk to cause frictional drag.

Detecting extrasolar planets: How did astronomers detect planets around nearby stars? The technique is called Doppler spectroscopy. The idea is this: we can see whether a star is moving toward or away from us by observing its spectrum and seeing whether the spectral lines are shifted to the blue or red, respectively, compared to the wavelength of the line as measured in Earth laboratories. If two stars are orbiting each other, then during part of the orbit, the first star will be moving away from us while the second star is moving toward us, while during the rest of the orbit, the motions are reversed. You can see this in the Binary star simulator, which also shows that the bigger the mass ratio (mass of heavy star/mass of lighter star), the less the heavy star moves compared to the lighter star. If the lighter star is faint, we may only be able to see the spectrum of the heavy star. We call such a system a single-line spectroscopic binary, in contrast to a double-line spectroscopic binary, where you can see the moving spectral lines from both stars. A star with a massive planet is just like a single-line spectroscopic binary: you can't see any light from the planet but you can see that some invisible object is orbiting the star because you can see the spectral lines from the star shifting periodically from red to blue and back again.

If the planet is in a circular orbit, the Doppler shift due to the star's motion oscillates according to a regular pattern called a sine wave. But some of the known planets are in eccentric orbits that plunge toward the star like comets. We recognize these eccentric orbits by the peculiar oscillation pattern of the Doppler shift. It is very challenging to find planets by the Doppler method because the star/planet mass ratio for these systems is typically about 1000, so the star's velocity is only about 1/1000 of the planet's velocity. In fact, the star's velocity variation due to the orbiting planet is less than +/- 100 meters/sec for some observed planetary systems. With such velocities, the fractional wavelength shifts of the spectral lines are tiny -- less than one part in 3 million. It is extremely difficult to measure such tiny wavelength shifts. Marcy and Butler have discovered more planets than anybody else because they built a spectrometer, using a very clever technique, that could measure wavelengths so accurately. But now many other people have built similar spectrometers and are observing many stars, so we can expect that the catalog of stars with known planets will have hundreds of entries within a few years.

The mass of the planet can be inferred from this periodic variation of the star's Doppler shift through Kepler's 3rd Law (as revised by Newton -- see Lesson 4). You will soon have the opportunity to see how this works in Homework 3. You will see, however, that the observations of the Doppler shift do not give quite enough information to tell the mass exactly. The uncertainty arises from our inability to measure the inclination, or tilt, of the planet's orbit. If the inclination angle, customarily denoted i, is 90^o, that means we see the orbit edge-on, while i = 0^o means pole-on. From the observations we can only infer the value of the product M sin i, and that product is listed in the Extrasolar Planets Encyclopedia. Since sin i is always less than 1, the mass of the planet must be greater than the value of listed in the catalog.

An Earth-like planet, with mass 300 times smaller than Jupiter, will only cause a star to move about 1 meter per second or less. We don't know how to build a spectrometer precise enough to measure such motions. So we don't expect to find any Earth-like planets in the near future, at least by this technique.

There is one exception to this rule, and that is the remarkable discovery of planets orbiting pulsars (neutron stars). As we discussed in Lesson 7, one pulsar has two confirmed planets, with masses (actually M sin i) equal to 2.4 and 3.8 Earth masses, and possibly two more. For details, see the Extrasolar Planets Encyclopedia. Because we can measure the frequency (hence Doppler shifts) of the radio pulses from pulsars much more accurately than we can measure the frequency of spectral lines from normal stars, we can detect planets of lower mass around pulsars than we can around normal stars.

It will take a long time to find planets in orbits similar to Jupiter's. The reason is simple: Jupiter takes about 12 years to orbit the Sun. We need to watch the spectral lines shift from red to blue over a full orbit to be sure that the star's motion is due to a planet. The present catalog doesn't list any stars with distant planets because astronomers haven't been observing the Doppler shifts with such high precision for that long. But now they are observing thousands of stars, so within a decade they will certainly find many planets with orbits more like Jupiter's.

Planetary Transits: In 1999, using a small telescope located in a parking lot in Boulder, astronomers at the High Altitude Observatory made the first detection of a planet by another method, called a planetary transit (see the STARE Project). If the planet's orbit is seen nearly edge-on, the planet will pass directly in front of the star and will partially eclipse the star's light, as illustrated below.

Just a few months ago, astronomers observed the same star (called HD209458) with the Hubble Space Telescope. They not only confirmed their earlier results, but also made the first detection of a gaseous atmosphere around an extrasolar planet. For details, see Hubble Makes First Direct Measurements of Atmosphere on World Around Another Star.

It's impossible to detect Earth-sized planets by the transit method using ground-based telescopes because the Earth's atmosphere causes a star's brightness to twinkle (scintillate) too much. A transit of an Earth-sized planet will diminish starlight by only a 5 - 40 parts in 100,000, much too slight to measure given this scintillation. But, viewed from a telescope in space, stars don't twinkle at all. Therefore, NASA is building a telescope in orbit called Kepler that is especially designed to detect transits of planets. Kepler can measure changes in starlight of only 1 part in 100,000, and is expected to discover hundreds of Earth-sized planets within a few years after it is launched in 2006.

Direct Imaging: Why not just take a picture of a planet? Well, that's very difficult. Consider, for example, the problem of imaging one of the planets listed in the catalog. For example the planet around the star 51 Peg. It has roughly the mass of Jupiter and an orbital radius of 0.05 Astronomical Units (1 AU is the distance from the Earth to the Sun). The star is at a distance of 15.4 parsecs, or about 47 light years. From these data I calculate that the maximum angle between the planet and the star is about 0.003 arcseconds. But the angular resolution of the Hubble Space Telescope is only about 0.1 arcsec, 30 times fuzzier! The problem is even worse than that: in optical light, the planet is roughly a million times fainter than the star. With existing technology, it's impossible to detect such a faint object so close to such a bright star.

How about imaging Earth-like planets? That would be fantastic, because it would be a big step on the way to finding out whether there is life on other worlds. It's the highest scientific priority of NASA. But the task is very difficult; it will require NASA to develop new technologies and to spend more money -- tens of $Billions -- than they ever have on a scientific project. (Maybe not as much as the Apollo project to land men on the Moon, but the primary mission of Apollo was not science.) NASA does in fact have a long-range plan for a mission called the Terrestrial Planet Finder to do this, and the European Space Agency (ESA) has a plan for a space observatory called Darwin to detect Earth-like planets.

Brown dwarf stars: A brown dwarf star, like the Sun, is a dense ball of hydrogen and helium gas; but unlike the Sun, it doesn't ever become hot enough in its center to burn hydrogen into helium. Theoretical calculations indicate that if a star has mass greater than about 80 times Jupiter's mass, it will begin to burn. Brown dwarf stars are less massive than that. On the other hand, Jupiter is also a dense ball composed mostly of hydrogen and helium, and we call it a planet, not a brown dwarf. The distinction between planets and brown dwarfs is subtle. It turns out that if the object weighs between 13 and 80 times Jupiter, it will be able to burn a rare isotope of hydrogen, called deuterium. But after a few million years, the deuterium is used up and the object ceases to burn. We call such a star a brown dwarf, but otherwise it's no different from a planet.

Astronomers suspect that there are many billions of brown dwarf stars in the Milky Way -- perhaps more brown dwarfs than normal stars. But brown dwarfs, even if they are burning deuterium at their centers, are very faint and hard to see. With a surface temperature of less than 1500 K, they would radiate very little optical radiation.

Nevertheless, in the past few years, astronomers have begun to find brown dwarf stars in abundance. The Extrasolar Planets Encyclopedia lists about a dozen that are orbiting normal stars. Most of these were found by Doppler spectroscopy, the same way as most extrasolar planets. But astronomers have succeeded in imaging a brown dwarf star in a binary system named Gliese 229B (the B means that the object is the binary companion of star GL229A in a catalogue of nearby faint red stars compiled by an astronomer named Gliese). GL229B has a mass more than 40 times Jupiter's mass and an orbital radius of about 40 AU -- roughly equal to the orbit of Pluto. The angular separation between GL229A and GL229B is about 6 arcseconds and the brightness contrast is not so great because GL229A is also very faint. In fact, GL229B was discovered first in images taken from the ground, although the image taken subsequently by the HST is better.

Very recently, astronomers have also found many brown dwarf stars that are not orbiting normal stars but are just floating free in interstellar space. They found the brown dwarf stars in an image of the Orion Nebula taken with a Near Infrared Camera (called NICMOS) on the Hubble Space Telescope. See Hubble Spies Brown Dwarfs in Nearby Stellar Nursery. For further details, see The Discovery of Brown Dwarfs, by Gibor Basri.

Update, March 19, 2001: Very recently, Japanese astronomers using an infrared camera on the Subaru Telescope have found a great number of brown dwarf stars and also a number of "floating small objects" that are not orbiting stars. Their masses appear to be a few times that of Jupiter, so they are too small to sustain fusion reactions. For further details and a spectacular picture, see Subaru Stares into a Cradle of Stars.

The NICMOS camera on the Hubble Space Telescope is no longer operating (it ran out of liquid helium coolant that is needed to keep its infrared detectors cool enough to operate). So we must wait a while to find out just how common brown dwarf stars are. But not long: the Space Infrared Telescope Facility (SIRTF), which NASA plans to launch in January 2003, will be far more sensitive than the Hubble Space Telescope for detecting nearby brown dwarf stars.

(Return to course home page)
Last modified October 19, 2002
Copyright by Richard McCray