The Milky Way and Beyond (12 page)

Spectroscopic binary stars are found from observations of radial velocity. At least the brighter member of such a binary can be seen to have a continuously changing periodic velocity that alters the wavelengths of its spectral lines in a rhythmic way. The velocity curve repeats itself exactly from one cycle to the next, and the motion can be interpreted as orbital motion. In some cases, rhythmic changes in the lines of both members can be measured. Unlike visual binaries, the semimajor axes or the individual masses cannot be found for most spectroscopic binaries, since the angle between the orbit plane and the plane of the sky cannot be determined. If spectra from both members are observed, mass ratios can be found. If one spectrum alone is observed, only a quantity called the mass function can be derived, from which is calculated a lower limit to the stellar masses. If a spectroscopic binary is also observed to be an eclipsing system, the inclination of the orbit and often the values of the individual masses can be ascertained.

An eclipsing binary consists of two close stars moving in an orbit so placed in space in relation to Earth that the light of one can at times be hidden behind the other. Depending on the orientation of the orbit and sizes of the stars, the eclipses can be total or annular (in the latter, a ring of one star shows behind the other at the maximum of the eclipse) or both eclipses can be partial. The best known example of an eclipsing binary is Algol (Beta Persei), which has a period (interval between eclipses) of 2.9 days. The brighter (B8-type) star contributes about 92 percent of the light of the system, and the eclipsed star provides less than 8 percent. The system contains a third star that is not eclipsed. Some 20 eclipsing binaries are visible to the naked eye.

The light curve for an eclipsing binary displays magnitude measurements for the system over a complete
light cycle. The light of the variable star is usually compared with that of a nearby (comparison) star thought to be fixed in brightness. Often, a deep, or primary, minimum is produced when the component having the higher surface brightness is eclipsed. It represents the total eclipse and is characterized by a flat bottom. A shallower secondary eclipse occurs when the brighter component passes in front of the other; it corresponds to an annular eclipse (or transit). In a partial eclipse neither star is ever completely hidden, and the light changes continuously during an eclipse.

The shape of the light curve during an eclipse gives the ratio of the radii of the two stars and also one radius in terms of the size of the orbit, the ratio of luminosities, and the inclination of the orbital plane to the plane of the sky.

If radial-velocity curves are also available—i.e., if the binary is spectroscopic as well as eclipsing—additional information can be obtained. When both velocity curves are observable, the size of the orbit as well as the sizes, masses, and densities of the stars can be calculated. Furthermore, if the distance of the system is measurable, the brightness temperatures of the individual stars can be estimated from their luminosities and radii. All of these procedures have been carried out for the faint binary Castor C (two red-dwarf components of the six-member Castor multiple star system) and for the bright B-type star Mu Scorpii.

Close stars may reflect each other's light noticeably. If a small, high-temperature star is paired with a larger object of low surface brightness and if the distance between the stars is small, the part of the cool star facing the hotter one is substantially brightened by it. Just before (and just after) secondary eclipse, this illuminated hemisphere is pointed toward the observer, and the total light of the system is at a maximum.

The properties of stars derived from eclipsing binary systems are not necessarily applicable to isolated single stars. Systems in which a smaller, hotter star is accompanied by a larger, cooler object are easier to detect than are systems that contain, for example, two main-sequence stars. In such an unequal system, at least the cooler star has certainly been affected by evolutionary changes and probably so has the brighter one. The evolutionary development of two stars near one another does not exactly parallel that of two well-separated or isolated ones.

Eclipsing binaries include combinations of a variety of stars ranging from white dwarfs to huge supergiants (e.g., VV Cephei), which would engulf Jupiter and all the inner planets of the solar system if placed at the position of the Sun.

Some members of eclipsing binaries are intrinsic variables, stars whose energy output fluctuates with time. In many such systems, large clouds of ionized gas swirl between the stellar members. In others, such as Castor C, at least one of the faint M-type dwarf components might be a flare star, one in which the brightness can unpredictably and suddenly increase to many times its normal value.

Near the Sun, most stars are members of binaries, and many of the nearest single stars are suspected of having companions. Although some binary members are separated by hundreds of astronomical units and others are contact binaries (stars close enough for material to pass between them), binary systems are most frequently built on the same scale as that of the solar system—namely, on the order of about 10 astronomical units. The division in mass between two components of a binary seems to be nearly random. A mass ratio as small as about 1:20 could occur about 5 percent of the time, and under these circumstances a planetary system comparable to the solar system is able to form.

The formation of double and multiple stars on the one hand and that of planetary systems on the other seem to be different facets of the same process. Planets are probably produced as a natural by-product of star formation. Only a small fraction of the original nebula matter is likely to be retained in planets, since much of the mass and angular momentum is swept out of the system. Conceivably, as many as 100 million stars could have bona fide planets in the Milky Way Galaxy.

Because planets are much fainter than the stars they orbit, extrasolar planets are extremely difficult to detect directly. Jupiter, for example, would be only one-billionth as bright as the Sun and appear so close to it as to be undetectable from even the nearest star. If candidate stars are treated as possible spectroscopic binaries, however, then one may look for a periodic change in the star's radial velocity caused by a planet swinging around it. The effect is very small—even Jupiter would cause a change in the apparent radial velocity of the Sun of only about 10 metres (33 feet) per second spread over Jupiter's orbital period of about 12 years at best.

Current techniques using very large telescopes to study fairly bright stars can measure radial velocities with a precision of a few metres per second, provided that the star has very sharp spectral lines, such as is observed for Sun-like stars and stars of types K and M. This means that at present the radial-velocity method normally can detect only massive extrasolar planets. Planets like Earth, 300 times less massive, would cause too small a change in radial velocity to be detectable presently. Moreover, the closer the planet is to its parent star, the greater and quicker the velocity swing, so that detection of giant planets close to a star is favoured over planets farther out.

Even when a planet is detected, the usual spectroscopic binary problem of not knowing the angle between the orbit plane and that of the sky allows only a minimum mass to be assigned to the planet. The first planet discovered with this technique was 51 Pegasi in 1995.

One exception to this last problem is HD 209458, a seventh-magnitude G0 V star about 150 light-years away with a planetary object orbiting it every 3.5 days. Soon after the companion was discovered in 1999 by its effect on the star's radial velocity, it also was found to be eclipsing the star, meaning that its orbit is oriented almost edge-on toward Earth. This fortunate circumstance allowed determination of the planet's mass and radius—0.69 and 1.42 times those of Jupiter, respectively. These numbers imply that the planet is even more of a giant than Jupiter itself. What was unexpected is its proximity to the parent star—more than 100 times closer than Jupiter is to the Sun, raising the question of how a giant gaseous planet that close can survive the star's radiation. The fact that many other extrasolar planets have been found to have orbital periods measured in days rather than years, and thus to be very close to their parent stars, suggests that the HD 209458 case is not unusual. There are also some confirmed cases of planets around supernova remnants called pulsars, although whether the planets preceded the supernova explosions that produced the pulsars or were acquired afterward remains to be determined.

The first extrasolar planets were discovered in 1992. More than 300 extrasolar planets were known by the early years of the 21st century, with more such discoveries being added regularly. Some of those studied have minimum masses of 40 or even 60 Jupiters, which means they are likely brown dwarfs.

Between 5 and 10 percent of stars surveyed have planets at least 100 times as massive as Earth with orbital periods of a few Earth years or less. Almost 1 percent of stars have such giant planets in very close orbits, with orbital periods of less than one week. In contrast, Jupiter, which has the shortest orbital period of any large planet (i.e., any planet more massive than Earth) in the solar system, takes nearly 12 years to travel around the Sun. Even the closest planet to the Sun, tiny Mercury, requires 88 days to complete an orbit. Models of planetary formation suggest that giant extrasolar planets detected very near their stars were formed at greater distances and migrated inward as a result of gravitational interactions with remnants of the circumstellar disks from which they accumulated.

The most massive planets that transit their stars are made primarily of the two lightest elements, hydrogen and helium, as are the Sun and its two largest planets, Jupiter and Saturn. Some of these planets seem to be distended in size as a result of heating by their stars. The lowest mass transiting planets contain larger fractions of heavier elements, as do the smaller planets within the solar system.

The majority of extrasolar planets with orbital periods longer than two weeks have quite eccentric (elongated) orbits. Within the solar system, the planets, especially the larger ones, travel on nearly circular paths about the Sun.

Stars that contain a larger fraction of heavy elements (i.e., any element aside from hydrogen and helium) are more likely to possess detectable planets. More massive stars are more likely to host planets more massive than Saturn, but this correlation may not exist for smaller planets. Many extrasolar planets orbit stars that are members of binary star systems, and it is common for stars with one detectable planet to have others. The planets detected so far around stars other than the Sun have masses from nearly twice to thousands of times that of Earth. Most, if not all, appear to be too massive to support life, but this too is the result of detection biases and does not indicate that planets like Earth are uncommon.

Research in the field of extrasolar planets is advancing rapidly, as new technologies enable the detection of smaller and more distant planets as well as the characterization of previously detected planets. Almost all the extrasolar planetary systems known appear very different from the solar system, but planets like those within the solar system would be very difficult to find around other stars with current technology. Thus, as more than 90 percent of those stars surveyed do not have detectable planets, it is still not known whether the solar system is normal or unusual. The U.S. National Aeronautics and Space Administration's Kepler mission was launched in 2009 and—using transit photometry from space, optimized to achieve unprecedented sensitivity for small planets with orbital periods of up to two years—aims to discover whether planets analogous to Earth are common or rare.