NASA - National Aeronautics and Space Administration

Origins Education Forum

Is there other life out there?

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 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? Do we represent the only life in the universe and, perhaps, the only intelligent life? 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 that could sustain life. 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 of galaxies in the universe and each of them is home to billions of stars. Surely, the argument 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.


The Orion Nebula contains young stars
around which planets may be forming.

While such disks might eventually evolve into planetary systems, are there any other planets existing today? Until 1994, we could not say for sure. Since then, there has been a flurry of planet detections. 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.


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!

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 a nuclear explosion is going off nearby. 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.

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 telescopes collect the light and route it 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 goals, 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 will be a 10-meter interferometer as part of the Space Interferometry Mission (SIM). Later, a much larger instrument (the length of a football field), designed to discover Earth-size planets and characterize their atmospheres for signs of life, will be deployed in the Terrestrial Planet Finder (TPF) mission.