4-7 Understanding how our planets formed around the Sun suggests that planets around other stars are common

If planets formed around our Sun, have they also formed around other stars? That is, are extrasolar planets orbiting distant stars just as our planets orbit the Sun? Although just a few years ago astronomers were not entirely sure that planets actually exist around other stars, today we have several lines of evidence convincing us that planets are common around other stars. To appreciate how remarkable these discoveries are, let’s consider the process that astronomers go through to search for extrasolar planets.

Directly Photographing Extrasolar Planets

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It is very difficult to make direct observations of planets orbiting other stars. The challenge is that planets are small and dim compared with stars; as seen by an alien astronomer far from our solar system, at visible wavelengths the Sun is 10⁹ times brighter than Jupiter and 10¹⁰ times brighter than Earth. A hypothetical planet orbiting a distant star, even a planet 10 times larger than Jupiter, would be easily lost in the star’s glare as seen through even the largest telescope on Earth. Despite these technical challenges, astronomers have been able to fine-tune telescopes to photograph, for the first time, a few planets visible orbiting distant stars. The first planets photographed are shown in Figure 4-18. The dramatic “first family” pictures of planets beyond our solar system are shown in Figures 4-18a and 4-18b, which were taken by the Gemini Observatory near the summit of Mauna Kea in Hawai’i. Figure 4-18c was taken by the Hubble Space Telescope and shows the motion of extrasolar planet Fomalhaut b. By 2013, about 30 planets beyond our own solar system have been discovered this way.

Figure 4-18: RIVUXG First Direct Images of Extrasolar Planets Orbiting Another Star Since exoplanets are typically lost in the overbearing glow of their host stars, directly observing exoplanets orbiting other stars is extremely challenging. (a) At a distance of about 500 ly away, the giant exoplanet visible in the upper left orbiting 1RXS is about 8 times more massive than Jupiter and orbits at a great distance—nearly 300 AU—from its star. (b) This image shows the first family of exoplanets ever captured, with three planets visible as tiny white dots with the bright central star blocked out. (c) This false-color composite image, taken from space with the Hubble Space Telescope, reveals the orbital motion of the planet Fomalhaut b following a 2000-year-long, highly elliptical orbit. The black circle at the center of the image blocks out the light from the bright star, allowing reflected light from the debris belt left over from formation of the planetary system and this planet to be photographed.

We have always suspected that planets might be a common by-product of the formation of stars when giant clouds of dust and gas collapse under gravity into a star and planetary system. The photographs of actual extrasolar planets orbiting other stars are a tremendous vindication of our theory of star and planetary system formation. It would also tell us that our own planetary system is not unique in the universe. Because at least one planet in our solar system—Earth—has the ability to support life, perhaps other planetary systems could also harbor living organisms.

COSMIC CONNECTIONS: Formation of the Solar System

Looking for Transiting Planets

The process of finding exoplanets by directly photographing them is an extremely difficult and time-consuming process. In other words, it isn’t very efficient. Instead, the vast majority of planets are found by indirect methods. One of the most surprisingly useful is called the transit method. This method looks for the rare situation in which a planet comes between us and its parent star, an event called a transit (Figure 4-19). As in a partial solar eclipse (Section 1-6), this causes a small but measurable dimming of the star’s light. If a transit is seen, the orbit must be nearly edge-on to our line of sight. As of May 2013, 308 extrasolar planets have been positively identified using ground-based telescopes. But that’s not all. Using the NASA Kepler space observatory, a space telescope specifically designed to look for transiting extrasolar planets, another 865 extrasolar planets have been positively identified, and another 2781 highly likely candidates have already been identified since its launch in 2007. Because only a small fraction of exoplanets might have orbits that line up the exoplanet precisely between their host stars and Earth, causing an observable transit, astronomers now estimate that there could be as many as 17 billion Earth-sized exoplanets in our Milky Way Galaxy alone.

Figure 4-19: Transiting Extrasolar Planet If the orbit of an extrasolar planet is nearly edge-on to our line of sight, we can learn about the planet’s (a) diameter, (b) atmospheric composition, and (c) surface temperature.

In addition to detection, there are at least three benefits of the transit method. One, the amount by which the star is dimmed during the transit depends on how large the planet is, and so tells us the planet’s diameter (Figure 4-19a). Another benefit is that a star’s light passes through the planet’s atmosphere while it is transiting and certain wavelengths are absorbed by the atmospheric gases. This absorption affects the spectrum of starlight that we measure and thus allows us to determine the composition of the planet’s atmosphere (Figure 4-19b). Finally, with an infrared telescope it is possible to detect a slight dimming when the planet goes behind the star (Figure 4-19c). That is because the planet emits infrared radiation due to its own temperature, and this radiation is blocked when the planet is behind the star. Measuring the amount of dimming tells us the amount of radiation emitted by the planet, which in turn tells us the planet’s surface temperature.

Measuring Stellar Doppler Shifts

The most powerful method available to astronomers for finding and confirming exoplanets is to search for stars that appear to “wobble.” If a star has a planet, it is not quite correct to say that the planet orbits the star. Rather, both the planet and the star move in elliptical orbits around a point that is the center of mass. As an example, consider that the Sun and Jupiter both orbit their common center of mass with an orbital period of 11.86 years. (Jupiter has more mass than the other seven planets put together, so it is a reasonable approximation to consider the Sun’s wobble as being due to Jupiter alone.) Jupiter’s orbit is about 5 times the Sun-Earth distance, 7.78 × 10⁸ km, while the Sun’s wobble is quite small, just 742,000 km, but detectable. The Sun’s radius is 696,000 km, so the Sun slowly wobbles around a point not far outside its surface. If alien astronomers elsewhere in the Galaxy were to detect the Sun’s slight wobbling motion in response to its gravitational attraction with Jupiter, they could tell that there was a large planet (Jupiter) orbiting our Sun. They could even determine the planet’s mass and the size of its orbit, even though Jupiter is small enough that it would be seemingly invisible.

Detecting the wobble of other stars is not an easy task, and we rarely actually see a distant star wobble back and forth because of an orbiting planet pulling on it. Instead, astronomers use the radial velocity method (Figure 4-20). This is based on the Doppler effect, which we described in Section 2-5. A wobbling star will alternately move away from and toward Earth. This will cause the dark absorption lines in the star’s spectrum to change their wavelengths in a periodic fashion. When the star is moving away from us, its spectrum will undergo a redshift to longer wavelengths. When the star is approaching, there will be a blueshift of the spectrum to shorter wavelengths. These wavelength shifts are very small because the star’s motion around its orbit is quite slow. As an example, the Sun moves around its small orbit at only 28 mi/h (45 km/h). If the Sun were moving directly toward an observer at this speed, the hydrogen absorption line at a wavelength of 656 nm in the Sun’s spectrum would be shifted by only 2.6 × 10⁻⁵ nm, or about 1 part in 25 million.

Figure 4-20: Detecting an Exoplanet Through Doppler Shift of Its Star The radial velocity method indirectly detects a tiny and nonvisible exoplanet gravitationally pulling on its host star during its orbit. When the star moves toward us because its planet is moving away from us, its spectrum is blueshifted, while it is alternatively redshifted when the star moves away from us as its planet moves toward us.

Detecting these tiny shifts requires extraordinarily careful measurements and painstaking data analysis, but it is tremendously fruitful. Since 1995, astronomers have used the radial velocity method to discover more than 500 exoplanets from ground-based observatories, and this same strategy is used to confirm suspected exoplanets identified by the transit method, described above.

Microlensing by Extrasolar Planets

In 2004 astronomers began to use a property of space discovered by Albert Einstein as a tool for detecting extrasolar planets. Einstein’s general theory of relativity makes the remarkable statement that a star’s gravity can deflect the path of a light beam just as it deflects the path of a planet or spacecraft. If a star drifts through the line of sight between Earth and a more distant star, the closer star’s gravity acts like a lens that focuses the more distant star’s light. Such microlensing causes the distant star’s image as seen in a telescope to become brighter. If the closer star has a planet, the planet’s gravity will cause a secondary brightening whose magnitude depends on the planet’s mass (Figure 4-21). Using this technique, astronomers have detected a handful of extrasolar planets at distances up to 21,000 ly and expect to find many more.

Figure 4-21: Microlensing Reveals an Extrasolar Planet (a) A star with a planet drifts across the line of sight between a more distant star and a telescope on Earth. (b) The gravity of the closer star bends the light rays from the distant star, focusing the distant star’s light and making it appear brighter. (c) The gravity of the planet causes a second increase in the distant star’s brightness.

This microlensing technique has only discovered a confirmed 19 exoplanets as of this writing, but this number came from observations covering only a very small portion of the sky. If 19 planets were discovered in just one small region of the sky, astronomers estimate that there could be more than 100 billion planets orbiting stars in our Galaxy alone—an average of about one planet per star, which is the largest estimate ever calculated (Figure 4-22).

Figure 4-22: Planets Orbit Most Stars This artist’s illustration shows how common it seems for planets to be orbiting stars throughout the Milky Way. In this illustration, exoplanets, their orbits, and their host stars are all vastly magnified compared to their real separations.