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The Search for Life in the Universe - Are We Alone?

We humans tend to believe that we occupy an extraordinarily unique and special place in the universe. Until a few hundred years ago, we believed that the entire universe revolved around Earth. When Copernicus argued instead that Earth orbits the Sun, we backed off, only to insist that if not Earth, then it must be our Sun around which the universe revolves. However, this view of a Sun-centered universe was also not supported by subsequent astronomical observations. In response, we believed that as a minimum the Sun is the center of our own galaxy, the Milky Way. But that was also not borne out by observation. Indeed, we continue to discover that our solar system's place in the universe is not unique at all.

But wait. What about life on Earth? Surely we represent the only life in the universe and, even more assuredly, the only intelligent life. Don't we? Actually, while we do not have concrete proof that there is life elsewhere in the universe, there is enough of a possibility to justify a search.

Looking for Planets

Before looking for life, we need to search for environments where life could be sustained. Where could such places be? Well, we know where they could not be. The millions of stars that we see in the night sky are hot gaseous bodies like our Sun. In fact, that is why they shine, which allows us to see them even though they are very far away. Life could not exist on these stars, just as it could not exist on our Sun. They are all far too hot. Then where? What if some of these stars have planets revolving around them just as our Sun has its "family" of nine planets revolving around it? If some of these planets are not too close to their sun (too hot) or too far away (too cold), and if they have the right life-sustaining properties, it is possible that we are not the only tenants of this vast universe.

If planet detection is a first step toward the search for life, what is the likelihood of finding planets outside our solar system? Until recently, the responses to this question were guesses based on statistics. For example, appealing to our intuition, Carl Sagan argued that there are "billions and billions" of galaxies in the universe and each of them is home to "billions and billions" of stars. Surely, the arguments went, among the universe's untold billions of stars, there must be others besides our not-so-special Sun that have planets. This reasoning, however appealing it may seem, is not hard scientific evidence.

But there are other scientific clues. Astronomers have recently learned that stars much younger than our Sun are surrounded by huge disks of gas and dust. This finding is significant because we believe that none of our solar system's current planets existed when our Sun was born 4.6 billion years ago. Instead, the young Sun had a disk — called a protoplanetary disk — around it. Over a few million years, the gas, rocks, and particles in the protoplanetary disk bumped into and stuck to one another, building up clumps that gave birth to the current planets, moons, comets, and asteroids in our solar system. So the presumption is that some disks we see around other stars might also evolve into planetary systems similar to our own. But how many stars have such disks around them? Recent observations of hundreds of young stars show that most of them have protoplanetary disks. For example, the photograph on this page, taken with the NASA Hubble Space Telescope, shows the Orion Nebula (large image) and within it a single disk around a young star (inset), silhouetted against the glowing gas of the nebula.

While such disks might eventually evolve into planetary systems, are there any other planets existing today? Until October 1995, we could not say for sure. But then a group of Swiss astronomers announced the detection of the first planet. Since then, there has been a flurry of similar detections by other teams. So it seems, after all, that our planetary system is not unique.

How Do You See a Planet next to Its Sun?

Since planets like Earth are very small, dim objects revolving close to their brilliant sun, it is hard to see these planets directly. Therefore, the method of detection so far has been indirect. Looking at some stars, astronomers have been noticing a very small, but precisely periodic wobble — a star being pulled to one side, then to the other. The most likely explanation is that there is a body revolving about such a "wobbling" star, and the gravitational tug of this body as it goes around the star pulls the star first to one side and then to the other.

Although indirect detections point to the existence of planets outside our solar system, we would like to see them directly. However, looking at a star and trying to see a small, dim planet next to it is like trying to see a firefly when nearby a nuclear explosion is going off. We need to block off the starlight. Even then, we need large space-borne telescopes to see Earth-sized planets around the stars.

The Hubble Space Telescope has a 2.4-meter mirror. To detect Earth-sized planets, we need a much larger telescope — more like the size of a football field. Clearly, we cannot afford to build and put in space a telescope this large. But NASA engineers and technologists are working on a better approach: rather than using a single, large telescope, they are working on a new optical space observational tool called an interferometer.

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A Universe Teeming with Stars

This image, captured by the NASA Hubble Space Telescope, is the deepest visual view of galaxies acquired to date. The telescope was pointed at a tiny patch of dark sky about the width of a dime held 23 meters (75 feet) away. Whereas images taken with ground-based telescopes show the patch to be nearly empty of stars or galaxies, this Hubble image reveals a field rich in distant galaxies.

Images such as this one indicate that there must be nearly 100 billion galaxies in the universe. With each galaxy containing hundreds of billions of stars, the total number of stars in the universe is on the order of 10 billion trillion! If only a small percentage of these stars had planets, imagine how many planets there could be!

An interferometer designed for operation in space would consist of a few rather small telescopes spaced along a long boom. The boom could be collapsed into a small volume to fit inside the shroud of a launch vehicle that would take it into space. Once in space, it would deploy to a full length of tens of meters. When the interferometer is pointed at a star, the light is collected by the telescopes and routed to a light combiner. After data processing, an image is produced. Such processing allows the interferometer to act as though it were a single telescope with a much larger area. This technique is commonly used today in ground-based radio astronomy.

NASA's Origins Program has as one of its principal goal the search for life elsewhere in the universe. To detect and characterize planets that may harbor life, the Origins Program will build and launch interferometers similar to those described above: first a 10-meter interferometer as part of the Space Interferometry Mission (SIM), and later a much larger instrument (the length of a football field) in the Terrestrial Planet Finder (TPF) mission designed to discover Earth-size planets and characterize their atmospheres for signs of life.



The Origins Education Forum is the central node for the education and outreach activities of the Origins Theme of NASA's Office of Space Science. The Origins Education Forum is managed for NASA by the Space Telescope Science Institute.

origins@stsci.edu
Last Modified: March 28, 2000