by Ben Bova
Moreover, the lopsided rotation of Uranus and the slow, retrograde rotation of Venus may have been caused by collisions with massive planetesimals early in the solar system’s history.
We know that Jupiter’s powerful gravitational field kicks asteroids out of the Asteroid Belt from time to time and deflects comets passing through the inner solar system. Computer studies have shown that, in the early stages of the solar system’s formation, Jupiter could have ejected embryonic planets out of the solar system altogether.
PLANETS WITHOUT STARS AND STARS THAT SWALLOW PLANETS
Computer simulations are one thing. Is there observational evidence to back it up? Yes. The Hubble Space Telescope has photographed some two dozen solitary bodies, huge dark objects somewhere between planets and brown dwarfs in mass, sailing through the Orion Nebula by themselves, not orbiting a star. They are visible against the bright background of the glowing nebula.
Theorist Boss believes these objects to be “sub-brown dwarf stars,” while other astronomers argue that they could be giant planets that were ejected from a young, dynamic planetary system.
Furthermore, in 2001, a Spanish-Swiss team of astronomers reported that the solar-type star HD82943 seems to have swallowed at least one of its planets! The star’s spectrum is unusually rich in lithium-6; the presence of this form of metal can only be explained by the star’s “ingesting” one or more of its planets. HD82943 has a hot Jupiter orbiting it; apparently the interactions that pushed that planet to its tight orbit also forced another giant planet to plunge into the star itself.
WORLDS IN COLLISION
This relatively new realization that planetary systems—including our own—are dynamic and sometimes dangerous yields an eerie echo of the ideas championed by Immanuel Velikovsky (1895–1979) in his 1950 book, Worlds in Collision.
A Russian-born psychoanalyst and amateur astronomer, Velikovsky attempted to show that the miraculous occurrences of the Bible’s Book of Exodus were the results of near collisions between the Earth and the planets Mars and Venus. Velikovsky believed Venus was a fragment of the planet Jupiter that was torn loose from the giant planet and established itself in its present orbit during the time of Moses and the great plagues visited upon Egypt.
The scientific community regarded Velikovsky as anathema, and astronomer Harlow Shapley even tried to keep his book from being published. To the general public, this gave Velikovsky the mantle of martyrdom, and he was compared to Galileo, championing ideas that were unpalatable to the “establishment.” Velikovsky’s supporters pointed out that his theory correctly predicted Venus’ high temperature and many other features of today’s solar system.
While current research and observations have shown that planetary systems are dynamic, astronomers scoff at the possibility that the planets were still careening through the solar system as recently as Biblical times, and there is no evidence to back Velikovsky’s beliefs. Yet the many books he wrote were extremely popular, and his basic idea of a dynamic solar system, once regarded as bizarre, is now an accepted part of scientific understanding.
IS THE SOLAR SYSTEM UNIQUE?
We come back to the old question: Is our solar system unique? The good news/bad news of extrasolar planets shows that many other stars harbor planets, but none of the planetary systems look much like our own.
By comparison with the hot Jupiters and planets flung out into interstellar space or swallowed up by their stars, our own solar system seems quite sedate. But is it unique? Are Earth-type planets—planets on which liquid water and life can exist—a rarity among the stars?
Appearances can be deceiving. The spectrographic technique for detecting planets through changes in a star’s radial velocity works best for planets that are very massive and orbiting very close to their stars. It would take alien astronomers at least a dozen years to track the orbit of Jupiter. Hot Jupiters can be nailed down in a few weeks because their “years” are only a few days long.
Like that scout straining to see what is on the other side of the chasm, the astronomers have found those extrasolar planets that are the easiest to detect: massive worlds orbiting close to their stars. This is like our scout spotting a herd of elephants on the other side of the canyon. Detecting mice will be much more difficult.
FINDING NEW EARTHS
Astrobiologists recognize that life is hardier and more tenacious than anyone had guessed even as recently as ten years ago. Yet they—and we—are still terribly constrained in what we know about life. We have only this one planet of ours as an example of a site where life can exist and flourish.
Perhaps there are forms of life that we cannot imagine thriving happily on hot Jupiters or brown dwarfs or the planets orbiting the dying stellar cores of pulsars. We have no way of knowing, as yet. The astrobiologists’ search for life has to be guided by what we do know, which means seeking life that is based on carbon-chain molecules and water.
Which means, in turn, that they want to locate Earth-type planets.
Realize that none of the extrasolar planets detected so far have been seen visually. With the exception of brown dwarfs, no “planetary companion” has been photographed. Even though most of them are much bigger than Jupiter, they are still too small, too dim, and too close to the glare of their parent star to be seen optically.
Late in 2001, however, a team led by David Charbonneau of the California Institute of Technology and Timothy M. Brown of the National Center for Atmospheric Research used the Hubble Space Telescope to probe the atmosphere of a planet orbiting the Sun-like star HD209458, which is about 150 light-years away. Although they did not directly image the planet, a gas giant slightly less massive than Jupiter, when the planet crossed in front of its parent star they analyzed the star’s light as filtered through the planet’s atmosphere. They found sodium in the planet’s 1,100°C atmosphere. Observations made in 2003 detected hydrogen in the planet’s upper atmosphere. Because of its high temperature, an estimated 10,000 tons of hydrogen per second is being boiled out of HD209458 B’s atmosphere. Future observations will look for methane, water vapor, and other components.
Still, it has been totally impossible to detect Earth-sized planets in any way with existing instruments and techniques. Jupiter is more than 300 times more massive than Earth, and all the planets detected so far are of Jovian mass or more.
How then can we find new Earths, assuming that they exist?
PLANET FINDERS
NASA’s answer is the Terrestrial Planet Finder (TPF), an array of four 3.5-meter telescopes that will be launched into a Sun-circling orbit to search for Earth-sized planets out to a range of fifty light-years. Although they will not be physically connected, the four telescopes will fly “in formation” and remain within about 100 meters of one another.
TPF will obtain imagery of extrasolar planets, even down to Earth-sized, through the process of interferometry, where overlapping images are combined to build up sharply detailed pictures. In addition, spectroscopes and other instruments will provide information on the planets’ atmospheric composition, temperature, size, and mass. Astrobiologists will want to look for evidence of free oxygen, carbon dioxide, or water vapor in a planet’s atmosphere.
NASA hopes to start launching TPF’s component telescopes in 2011. However, all NASA programs depend on government funding, and Congress has often cut back or eliminated altogether programs that it considers wasteful or overly ambitious. It is a constant, year-to-year battle to keep long-term programs going, and the bigger the program, the more likely it is to attract the axes of Congressional budget-cutters. TPF is in the billion-dollar range and could be quite vulnerable. As we have seen in the case of SETI, some politicians find it easy to gain publicity for themselves by loudly proclaiming that they are saving the taxpayers’ money by chopping programs that have obvious “giggle factors.”
NASA’s Jet Propulsion Laboratory is running the TPF program. JPL has been responsible for space science programs such as the Pioneer and Voyager probes of
Jupiter and the outer planets, and the Viking, Pathfinder, Global Surveyor, and other landers and orbiters studying Mars.
NASA also plans to launch in 2009 the Space Interferometry Mission (SIM), which will put into orbit three pairs of small telescopes mounted on a 10-meter-long boom to measure the slight shifts in stars’ radial velocities. Capable of detecting planets of five to ten times the Earth’s mass, SIM is being developed by Lockheed Martin Corporation and TRW.
Other nations are planning missions to find extrasolar planets, too. Canada’s Microvariability and Oscillation of Stars (MOST) was scheduled for launch on a Russian booster in early 2003. France’s Corot and the European Space Agency’s Eddington satellites will also search for planets transiting across their stars’ faces but only as secondary tasks to other astronomical observations.
TWINKLE, TWINKLE LITTLE STAR . . .
Meanwhile, at NASA’s Ames Research Center, a more modest program aimed at detecting Earth-sized extrasolar planets is quietly underway. The Kepler program has already detected an Earth-sized planet of another star—in a laboratory simulation.
Kepler is based on the idea of detecting the dip in a star’s brightness when a planet crosses in front of the star’s face. Kepler’s originator and manager, astronomer William J. Borucki, believes that his system will be able to detect Earth-sized planets out to a range of 2,000 light-years.
Although no one has yet photographed an extrasolar planet, the concept of detecting one by measuring fluctuations in a star’s brightness has been successfully accomplished. The first visual detection of a planet orbiting another star was made on November 7, 1999. Note that “visual detection” does not mean “seen.”
Gregory W. Henry of Tennessee State University in Nashville used a small, automated telescope at the Fairborn Observatory in Arizona’s Patagonia Mountains to measure the dip in brightness of the star HD209458 as its Jupiter-sized planet passed in front of it. This is the same planet whose atmosphere was later measured by Charbonneau and Brown.
HD209458’s planet was originally detected by the Marcy and Butler team, using the Doppler-shift radial velocity technique. Using photoelectric equipment (which converts incoming light to an electrical signal), Henry and his colleagues detected the star’s change in brightness and used that information to deduce some details about this extrasolar world.
From the star’s 1.7 percent decrease in brightness during the planet’s transit across its face, Henry concluded that the planet’s radius is 1.6 times larger than Jupiter’s. By determining the inclination of its orbit, Henry was able to calculate that its mass is only 63 percent of Jupiter’s, which leads to the conclusion that the planet must be largely gaseous, like the Jovian planets in our own solar system.
In January 2003, Dimitar D. Sasselov and colleagues of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, announced that they used the light-dimming technique to detect a planet 5,000 light-years from Earth, some thirty times farther than any previous extrasolar planet. The planet, which is about 90 percent of Jupiter’s mass, is so close to its parent star that its “year” is only twenty-nine hours long. It is so hot that molten iron may form in its gaseous atmosphere. When it transited in front of its star, the planet blocked about 1 percent of the star’s light.
These successes led Borucki and his team at Ames to believe that Kepler can find terrestrial-sized planets. In a laboratory mockup of the system, the Kepler equipment has already detected the equivalent of Mars-sized planets, half the diameter of Earth.
Basically, the Kepler system consists of a 1.4-meter Schmidt reflector telescope linked to photoelectric detectors capable of measuring differences in light input down to one part in a hundred thousand. When an Earth-sized planet passes in front of a star of the Sun’s brightness, the drop in light output is 0.008 percent, or eight parts in a thousand—well within the capability of Kepler’s photometers.
None of Kepler’s components is exotic equipment, Borucki points out. It is all off-the-shelf, commercially available hardware. Instead of observing one star at a time in a tedious “island hopping” technique, Kepler’s wide-angle telescope will be able to observe 100,000 stars simultaneously, gathering data for up to four years before moving to another section of the sky. This would be impossible on Earth, where our planet’s rotation makes the stars rise and set every twenty-four hours, but Kepler’s Sun-circling orbit will allow such long-term observations.
The photometers measure the drop in light when a planet transits across the face of its parent star. From space, above Earth’s obscuring atmosphere, Borucki believes that the Kepler system can find Earth-sized planets out to a distance of 2,000 light-years. There are tens of thousands of Sun-like stars within that radius.
Of course, not all those stars will actually have planets orbiting around them. Current thinking, though, is that at least half the stars are accompanied by planetary systems. Even so, the planets have to be oriented to their parent star so that their orbits cross the star’s disk, as seen from Earth. That cuts down the chances of discovery even more, but it probably still leaves thousands of potential “hits.”
Kepler will not provide an image of the planet; it merely detects the decrease in light when the planet transits its parent star. But such detection will also provide vital information on the planet’s orbital period, its distance from the star, and its size. From that information, astronomers can calculate the planet’s mass and temperature to determine if the planet could have liquid water on its surface: a requirement for life.
Kepler is a relatively small program, projected to cost $299 million, including the cost of a Delta II rocket booster. The 903-kilogram spacecraft is currently scheduled to be launched into a Sun-circling orbit in 2006.
KEPLER VS. TPF
What effect will Kepler have on Terrestrial Planet Finder? Politicians in Washington might see Kepler as a cheap way to kill TPF. The politics within NASA and between NASA, the White House, and Congress can get rough.
Borucki sees no conflict between the two programs. “Kepler will show TPF where to look,” he says. Kepler can find Earth-sized planets, TPF can then focus on them to glean more detailed data. Working together, the two programs can provide much more information for astrobiologists than either program could individually. Indeed, if and when Kepler begins to detect Earth-sized planets, the scientific community—and the taxpaying public—will want to learn more about those worlds. The political pressures would then undoubtedly work in favor of going ahead with the Terrestrial Planet Finder.
18
The Heartbreak of SETI
The truth is out there.
—From the television series The X-Files
THE IDEA OF LISTENING for radio signals from other worlds is as old as radio itself. In 1901, Nikola Tesla (1856–1943), the Croatian-born inventor and pioneer researcher in electricity, believed he had been the first to hear “the greeting of one planet to another” with the radio equipment he had built for himself. He thought he would be able to win the Guzman Prize for finding extraterrestrials (Chapter 2) by making radio contact “with the stars.”
Nineteen years later, Guglielmo Marconi (1874–1937), the father of wireless radio, told an interviewer that he hoped radio messages could be transmitted to the stars. In 1924, as we have seen, a nationwide effort was dedicated to picking up radio messages from Mars. It was unsuccessful.
As discussed in Chapter 3, Cocconi and Morrison’s seminal 1959 paper, “Searching for Interstellar Communications,” ended with the words:
The probability of success is difficult to estimate, but if we never search the chance of success is zero.
Unfortunately, after more than four decades of searching for alien signals with radio telescopes, SETI investigators still have achieved zero success. SETI, the Search for Extraterrestrial Intelligence, has followed a frustratingly hard and bumpy path.
IS THERE A NEEDLE IN THE HAYSTACK?
The problem that SETI investigators face is ga
rgantuan. Finding a needle in a haystack is simple, by comparison.
The needle: Our Sun is a member of the Milky Way galaxy, a vast pinwheel of stars and clouds of dust-laden gas, some 100,000 light-years in diameter. Out of the hundred billion or more stars of the Milky Way galaxy, which ones might hold planets that harbor intelligent civilizations? Even if the radio astronomers restricted their search to Sun-like stars, that still leaves billions of possibilities.
The haystack: Which frequencies would an alien civilization be using? We are accustomed to tuning our radio dials to perhaps two or three dozen local stations. Each station broadcasts on a certain frequency. There are approximately 10,000 radio stations in the United States. Suppose you were on the Moon and you wanted to listen to a specific station (say, WGCU-FM in southwest Florida), but you didn’t know which frequency it uses. How would you find that particular station? You would have to tune your radio receiver to one frequency after another until you found the one you wanted.
There are millions upon millions of possible radio frequencies that alien broadcasters might be using. How do you find a frequency on which they are broadcasting? SETI investigators must pick a target star, then sift through as many frequencies as possible, and then go on to the next candidate star and go through the same procedure, time and again. Much of the work in radio SETI has been devoted to developing electronic equipment that can sift through millions of frequencies quickly, so that the radio telescope may be moved from one star to another in a reasonable time.