8-5 Searches with space-based infrared telescopes and Earth-based radio telescopes for Earthlike planets and alien civilizations are under way

In Chapter 4, we discussed the numerous techniques for finding extrasolar planets orbiting other stars. We also considered the technical challenges that present barriers to finding planets orbiting distant stars. Let’s briefly reconsider the most profitable search so far, the search for transiting planets.

Searching for Extrasolar Planets

The most fruitful technique for finding extrasolar planets beyond our own solar system has been the one proven by NASA’s Kepler mission, which has been successful in finding thousands of planets. The technique works like this: If a star is orbited by a planet whose orbital plane is oriented edge-on to our line of sight, once per orbit the planet will pass in front of the star in an event called a transit. This causes a temporary dimming of the light we see from that star. When using a telescope with a wide field of view, projects like Kepler are able to continuously monitor thousands of stars at once and find hundreds of planets in a very short time.

Much more interesting is to actually take direct images and photographs of extrasolar planets. If we are looking for extraterrestrial life, we sure would like to have an actual picture. Planned for a launch after 2015, Darwin is a European Space Agency telescope that will search for Earthlike planets by detecting their infrared radiation. The rationale is that stars like the Sun emit much less infrared radiation than visible light, while planets are relatively strong emitters of infrared. Hence, observing in the infrared makes it less difficult (although still technically challenging) to detect planets orbiting a star.

The Darwin telescope will also analyze the infrared spectra of any planets that it finds, in the hope of seeing the characteristic absorption of atmospheric gases such as ozone, carbon dioxide, and water vapor (Figure 8-11). The relative amounts of these gases, as determined from a planet’s spectrum, can reveal whether life is present on that planet.

Figure 8-11: The Spectrum of a Simulated Planet The image on the left is a simulation of what infrared telescopes might see when looking for habitable planets. The white dot at the center is a nearby Sunlike star, and the smaller dots around it are planets orbiting the star. On the right is the simulated infrared spectrum of one of the planets, showing broad absorption lines of water vapor (H₂O), ozone (O₃), and carbon dioxide (CO₂). While all these molecules can be created by nonbiological processes, the presence of life will change the relative amounts of each molecule in the planet’s atmosphere. Thus, the infrared spectrum of such planets will make it possible to identify worlds on which life may have evolved.

The Darwin telescope will need to achieve enough resolution to detect individual planets. One proposed mission design makes use of interferometry. By combining the light from three widely spaced dishes, each at least 3 m in diameter, Darwin will make the sharpest infrared images of any telescope in history. NASA has proposed a similar planet-finding interferometry mission, called Terrestrial Planet Finder, but the funding for this mission is uncertain.

A more speculative project is an infrared telescope with sufficient resolution that some detail would be visible in the image of an extrasolar planet. One concept for such a mission would consist of five Darwin-type telescopes flying in a geometrical formation some 6000 km across (equal to the radius of Earth). All five telescopes would collect light from the same extrasolar planet, and then reflect it onto a single mirror. The combined light would go to detectors on board a sixth spacecraft. The technology needed for such an ambitious mission does not yet exist but may become available within a few decades.

Sometime during the twenty-first century, missions such as Darwin may answer the question, “Are there worlds like Earth orbiting other stars?” If the answer is yes, radio searches for intelligent signals will gain even more impetus.

Radio Searches for Alien Civilizations

If other civilizations are trying to communicate with us using radio waves, how can we know how to tune our radio telescopes? This is an important question, because if we fail to tune our radio telescopes just right, we might never know whether the aliens are out there.

A reasonable choice would be a frequency that is fairly free of interference from extraneous sources. SETI pioneer Bernard Oliver was the first to draw attention to a range of relatively noise-free frequencies in the neighborhood of the microwave emission lines of hydrogen (H) and hydroxide (OH) (Figure 8-12). This region of the microwave spectrum is informally called the water hole, not because we are looking for water, but because H and OH together would form the letters H₂O, or water.

Figure 8-12: The Water Hole This graph shows the background noise level from the sky at various radio and microwave frequencies. The so-called water hole is a range of radio frequencies from about 10³ to 10⁴ megahertz (MHz) in which there is little noise and little absorption by Earth’s atmosphere. Some scientists suggest that this noise-free region would be well suited for interstellar communication. Within this region dubbed the water hole, the principal source of noise is the afterglow of the Big Bang, called the cosmic microwave background. To put this graph in perspective, a frequency of 100 MHz corresponds to 100 on an FM radio, and 10³ MHz is a frequency used for various types of radar.

In 1989, NASA began work on the High Resolution Microwave Survey (HRMS), an ambitious project to scan the entire sky at frequencies spanning the water hole from 10³ MHz to 10⁴ MHz. HRMS would have observed more than 800 nearby solar-type stars over a narrower frequency range in the hope of detecting signals that were either pulsed (like Morse code) or continuous (like the carrier wave for a TV or radio broadcast). The sophisticated signal-processing technology of HRMS would have been able to sift through tens of millions of individual frequency channels simultaneously. It would even have been able to detect the minute Doppler shifts in a signal coming from an alien planet as that planet spun on its axis and moved around its star.

Sadly, just one year after HRMS began operation in 1992, the U.S. Congress imposed a mandate requiring that NASA no longer support HRMS or any other radio searches for extraterrestrial intelligence. This decision, which was made on budgetary grounds, saved a few million dollars—an entirely negligible amount compared to the total NASA budget. Ironically, the senator who spearheaded this was from the state of Nevada, where tax dollars have been spent to signpost a remote desert road as “The Extraterrestrial Highway.”

Even though U.S. government funding is no longer available for this search, several teams of scientists remain actively involved in SETI programs. Funding for these projects has come from nongovernmental organizations such as the Planetary Society and from private individuals. Since 1995 the SETI Institute in California has been carrying out Project Phoenix, the direct successor to HRMS. When complete, this project will have surveyed a thousand Sunlike stars within 200 ly at millions of radio frequencies. At Harvard University, BETA (the Billion-channel ExtraTerrestrial Assay) is scanning the sky at even more individual frequencies within the water hole. Other multifrequency searches are being carried out under the auspices of the University of Western Sydney in Australia and the University of California.

A major challenge facing SETI is the tremendous amount of computer time needed to analyze the mountains of data returned by radio searches. To this end, scientists at the University of California, Berkeley, have recruited nearly 5 million personal computer users to participate in a project called SETI@home. Each user receives actual data from a detector called SERENDIP IV (Search for Extraterrestrial Radio Emissions from Nearby, Developed, Intelligent Populations) and a data analysis program that also acts as a screensaver. When the computer’s screensaver is on, the program runs, the data are analyzed, and the results are reported via the Internet to the researchers at Berkeley. The program then downloads new data to be analyzed. SETI@home has provided as much computer time as a single computer working full time for 3 million years! All current SETI projects make use of existing radio telescopes and must share telescope time with astronomy researchers. The SETI Institute is working to build and put into operation a radio telescope that will be dedicated solely to the search for intelligent signals. This telescope, called the Allen Telescope Array, will eventually be hundreds of relatively small and inexpensive radio dishes working together. Perhaps this new array will be the first to detect a signal from a distant civilization.

Go to Video 8-3

The potential rewards from such searches are great. Detecting a message from an alien civilization could dramatically change the course of our own civilization, through the sharing of scientific information with another species or an awakening of social or humanistic enlightenment. In only a few years our technology, industry, and social structure might advance the equivalent of centuries into the future. Such changes would touch every person on Earth. Mindful of these profound implications, scientists push ahead with the search for extraterrestrial intelligence.