Astronomer*
One argument is that the basic laws of science are the same everywhere and that there are enough different combinations due to chance to allow life to arise more than once.

Douglas Scott
Professor
University of British Columbia
The Universe is a big place. There are something like 10,000,000,000,000,000,000,000 stars in the part of the Universe observable to us. So the chances of life would have to be very small in order for there to be nothing else out there. On the other hand, some people have argued that since we haven't seen any evidence yet of life elsewhere, that perhaps there is none.

Dr. Michael M. Davis
Astronomer
SETI Institute
Most of the discussion is based on how it would be for the earth, out of billions of possible planets, to be the only place where life formed. The fact that life formed here almost immediately after the water in the oceans stopped boiling is pretty impressive evidence that the initial emergence of life may be very general and almost unavoidable. However, whether CELLULAR life, with the explosive growth of different varieties of biological species, which occurred on earth VERY much later ('recently' on an astronomical time scale) has occurred elsewhere is very much more debatable. Perhaps this was a unique accident of evolution on earth, perhaps not. What a casual way to answer such a crucial question! But in fact, we just don't know.

Yervant Terzian
Professor
Cornell University
The laws of the universe are universal! Given the same conditions the same things happen. And there should be many similar places as the earth in the universe.

Prof. Wayne G. Roberge
Theoretical Astrophysicist
Rensselaer Polytechnic Inst.
Many nonscientists think that aliens must exist. Their thinking goes like this: Our Milky Way galaxy has 100 billion stars. That's so many stars that life must have started on some of them.

If the probability for life to get started in larger than one in a hundred billion, this argument is correct. However, we just don't know what that probability is! The problem is that we haven't figured out the sequence of events that started life here on Earth. Once we know that, we'll know the probability.

My research is in this area. I am one of the scientists in the New York Center for Studies on the Origins of life. We're trying (with many other scientists around the world) to piece together the events that led to the first organisms on Earth. Lots of progress is being made. For example, we now know that planets (which are necessary for life) exist outside our own solar system. Until 1995, no one was sure about this.

Karen Vanlandingham
Assistant Professor
Columbia University's Biosphere 2 Center
I have yet to see any evidence that I'd believe.

Astronomer*
Carl Sagan once made the point in a book about extraterrestrial life that all the isolated sightings of aliens by one or two people carry very little weight compared to a single sighting by a large number of people. Until this happens, there is no argument; the evidence does not support the idea, as much as I would like to believe it.

Eilat Glikman
Graduate Student
Columbia University
From flying-saucers to shape-shifters on the x-files, I guess the arguments are pretty varied. I am not an expert on this subject, but I know of no evidence to support a theory of extraterrestrial life.

Professor*
This really requires a long answer, but the short
summary is this:

Life probably exists in many places in the Universe. The speed (and hence, we infer, the ease) with which it formed on Earth suggests that it forms fairly easily.

Intelligent life has not yet proven to be a successful experiment, if longevity is the success criterion. It took almost 4 billion years for intelligence to develop on Earth. It may be modestly common or it may not. There is no very reliable basis for an educated guess. Speaking personally, I would be surprised if intelligent life has not developed elsewhere.

Walter Harris
Astronomer
University of Wisconsin-Madison
Most of the arguments in favor of alien life are numerical. There are 100 billion stars in our galaxy, and we know that many of them have planets. Thus, unless life is *extremely* unlikely to develop, the odds are in favor of a living cosmos. Of course, we don't actually *know* what the likelihood of making living things is, so there are holes in this argument. Indeed, many scientists who don't believe in life elsewhere use the *same* numerical arguments to prove their point!

Things get even more muddled when discussing intelligent life. We have far less idea how common tool using life forms such as ourselves are or how long they survive. My gut says that we aren't alone in this regard, but that the universe probably isn't exactly crawling with Klingons and Wookies either. We are looking though, and have not yet heard anything. Given the scope and brevity of our efforts to date, scientists differ about what the silence means. Still, it may be that in your lifetime we will be able to say one way or another whether anyone out there is transmitting in our direction.

Steven Balbus
Astronomy Professor
University of Virginia
There is no compelling evidence.

Professor Rex A. Saffer
Physicist, Astronomer, Educator
Dept. of Astronomy & Astrophysics, Villanova University
There aren't really any good ones that I know of!

Bob Mathieu
Professor of Astronomy
Department of Astronomy, University of Wisconsin - Madison
I presume you mean aliens that might come to Earth? In some ways the arguments here are much the reverse of the ones in answering Cassy's question, for aliens would have to solve the same problems. Perhaps we will be fortunate to learn from them, although it might be a bit like trying to teach calculus to an earthworm!

Astronomy Professor*
There are numerous arguments but three broad categories are: 1. there are no aliens because aliens are not common, 2. there are no aliens because they do not leave their home worlds either because they are uninterested in interstellar travel or they destroy themselves before achieving it, and 3. there are many aliens but they are deliberately avoiding letting us know of their existence.

John Huchra
Astronomer/Professor
Harvard-Smithsonian
There's one big one -- if the Universe is infinite, aliens *must* exist. But they can be pretty darn far away and may not be likely to contact us. That's especially true if the Universe has a finite age --- the aliens may live outside our mutual horizons (see #4).

*Respondents opted for anonymity and we respect their wishes.