Questions and Answers about Life in the Universe

1. I was reading about the Drake equation and was wondering: could it be completed with today's knowledge and  technology?  If not, how much of it are we able to complete?

A little bit is known about the first two factors in the Drake Equation, N* and fp.  The rest of the factors are anybody's guess at this point.

The number of stars in our Galaxy, N*, has been estimated fairly accurately -- it's around 200 billion.  In the last few years, searches in the nearby part of the Galaxy have found planets orbiting a few percent of the stars which are similar to our Sun.  About 20% of the nearby stars are similar in mass and luminosity to the Sun, so the fraction of stars with planets, fp, might be roughly 1%.

As to the fraction of planets which support life, we have no information at present.  A network of space telescopes planned by NASA and called the Terrestrial Planet Finder may be able to detect ozone in the atmospheres of Earth-like planets around the nearest stars.  An atmosphere containing ozone would suggest life similar to the kind we know on Earth.  Some difficult engineering problems will have to be solved before this network of telescopes can begin work.  Observations might start as early as ten years from now.

Since we know of only one planet where life and intelligence have arisen, it is probably too early to make firm statements about the fraction of planets ready to communicate with us.

Neal Turner

2. For some people this is a sensitive question and if you don't want to answer that is fine but I want an astronomer's point of view.  I do think there is life out there, somewhere, and I just want a professional's opinion.  If we ever find life, do you think it will be intelligent?  Do you think they would be hostile?  Do you think they'd contact us first?  Would they be close, like in our galaxy, or would they be far away, like in another universe?  Would they look like us, or like some other weird thing?

Your question is a really good one and one which most people today would love to know the answer to.  We have not, as of yet, found life elsewhere in the Universe, this does not mean that life could not be there. I think that there is a very good chance that simple life (i.e. bacteria) could exist elsewhere.  Some astronomers are even hopeful that the ice moon of Jupiter, Europa, may have bacteria in oceans beneath its surface. And there are very cool species on Earth called extremophiles which seem like they could live in some of the extreme conditions we see on other worlds.   As for intelligent life, I think most people, myself included, would like to think that we are not alone in the Universe.  And when you think about the number of stars in a galaxy and the number of galaxies in the Universe it is hard to think that at some point other intelligent beings did not exist.  But we have no idea what the life span of any civilizations may be. The time may be too short for any sign of them to reach us (the distances between stars are really really huge) before they died out.  So we may never meet other intelligent life forms even if they do exist.

I hope this answers your question.  You may want to look up something about the Drake Equation, it may interest you, and it deals with life in the Universe in an interesting way.  And you may also want to check out the SETI webpage at  You may find some other opinions there.

Mandy Proctor

3. Is other than Mars is there any other prospects of life that astronomers are looking at.  Also are there any web sites that you have or know of that deal with matters like these. As I see it, there is no possible way that life can not exist out there in the universe. If we can't account for more than 90% of our universe, then how can we know that there isn't life? We just haven't discovered it. But my question is, is there any way, whatsoever we could communicate with another galaxy.  In the movie "Contact" radio signals were sent, is that possible?

Since your questions are related, I'm going to answer them together. My answer is kind of long, hope you don't mind!

The short answer to the second question "How can we know there isn't life?"  is "We can't."  That's why astronomers are still looking. Eventually we may have looked long and hard enough to decide the answer is "There probably isn't life outside Earth." But "life" and "intelligent life capable of communicating with us" are two very different things.  We may someday find algae in the Martian permafrost, but I doubt they'd have much to say.

In our search for evidence of life beyond Earth, we have a natural bias to start with places where there may be water or water may have existed in the past.  As Scott alluded to, some think we may find fossils of ancient life on Mars -- where we believe water was once (but no longer is) abundant.  A few years ago there was a buzz when some scientists believed they found micro-organic fossils in a piece of a Martian meteorite.  The other place in our Solar System that astronomers are hopeful about is Europa, one of Jupiter's giant moons.  Europa is covered by a crust of ice and there are indications (for instance, stress fractures in the ice) that the interior of Europa is heated. So, somewhere beneath the ice may be liquid water, and thus the possibility of little beasties.

Now let's talk about communication. The first thing to remember is that we can only talk so fast. We can only send signals at the speed of light, and space is big, so it takes a long time for a signal to reach its destination. For instance, the nearest star, Proxima Centauri, is 4.25 light-years away. So if we sent a message to someone at Proxima Centauri and she replied immediately, it'd be 8 and a half years before we received the reply. These kinds of distances make it difficult to carry on a conversation.  (Plus how do we know which star to send a signal to?).  The nearest galaxy like our own, Andromeda is over 2 million light-years away -- so there is really no point in sending signals to other galaxies.

Listening for signals from nearby Sun-like stars (which we hope would have planets circling them) is a better idea. In fact, this is what they were doing in the movie "Contact" -- listening not sending. There is a large listening project that has been going on for many years. It is called SETI -- the Search for Extraterrestrial Intelligence.  SETI can monitor many stars at many frequencies and sift through the gobs of data for signals. (Nothing so far, but keep your fingers crossed).

Dr. Marc Pound

4. I was wondering what your opinion about life on other planets.... Do they exist? What do they look like?

Life in the Universe… that's a very interesting and philosophical topic.  If you ask me whether I believe that extraterrestrial life exists, then I would have to say "no".  I'm of the opinion "I'll believe it when I see it".  But if you ask me is there a possibility that extraterrestrial life exists, then I would say "yes".  The universe is a big, big place and very, very old. It seems very unlikely in the last... oh, about 14 billion years that there has not been life forms existing on another planet. Currently, there are about 50 extra-solar planets that have been discovered.  And these discoveries were made within the past couple of decades.  So there are many, many more planets out there and surely one of them must be able to sustain life.

But that brings us to the question of what are the conditions of sustaining life.  To us humans’ condition of life means liquid water, oxygen, and warm climate.  But that's not necessarily true.  We know that there's life at some of the most hostile environments imaginable, such as in the great depths of the ocean.  In the deep parts of the ocean, it's dark, cold, and has great pressure.  We can't live there, but we know life exists there.  These life forms look very different than us.  That has evolved to adapt to conditions in their environment.  So it is very hard to say what aliens would look like.  They may or may not look like the generic big-eyed aliens with big heads and skinny bodies.  What they look like depends on their environment.  For instance, our eyes are detectors of electromagnetic radiation (light) and different sized eyes are sensitive to different wavelengths.  Bigger eyes are sensitive to longer wavelengths.  So if life had evolved on a planet orbiting an O star (which emits more short wavelengths than the Sun) instead of the Sun (a G star), these life-forces would have smaller eyes than we do.  On the other hand, if life had evolved near a star that is a K star (which emits more longer wavelengths of light) instead of the Sun, then these life forms would have bigger eyes.  Therefore, it is very hard to say what ETs look like since we don't know the conditions of their home planet. So until we see an alien for ourselves, we can only use our imagination to think about what extraterrestrials would look like.

Stacy Teng

5. We have learned that all of the elements through iron form after average sized stars explode and how the rest of the elements form after giant stars supernova. Some of these are the elements responsible for life on our planet. I have a theory that if the conditions are right, life on other planets would be just like the life on our own.  I was wondering if there were any scientific theories like mine that had been developed by astronomers?

There have been a lot of theories by astronomers about what life might be like on other planets, but nobody knows for sure. Most astronomers think that life would be carbon based (that is, made from organic compounds, which all contain carbon), although some people have conjectured that it might be silicon based (because silicon has the same number of outer electrons as carbon, its just one row below it on the periodic table). All life on Earth is carbon based, so most astronomers think that other life would have to be carbon based as well.

Beyond the basic chemical makeup of extraterrestrial life, it is really hard to predict what life might be like on another planet. Since most life we know of on Earth has 2 or 4 legs, its easy to speculate that all life would be like that. This is very popular in science fiction, notice almost all the aliens in Star Wars have 2 legs and are "humanoid", however it doesn't have to be this way. For instance, imagine life forming on a planet which is completely covered in water, then walking on two feet wouldn't be very helpful, so life there might all have fins or tails of some kind. Another example would be if life formed on a gaseous planet, then there might not be any need to walk or swim, but only to float or fly. So on that planet, life might have wings or be very thin and stretched like a piece of paper in order to best survive.

In general, most astronomers would probably say that life on another planet would be similar to life on Earth in several ways: It would probably be carbon based. It would require energy to live (in our case the "energy" is food, but for plants it is sunlight). It must be able to reproduce. Finally, for us to consider it to be "sentient" it probably would need some way to move, whether that be via legs, fins, wings, or something completely new. With all that said, the chances of that life looking very similar to humans or any other animal on Earth are probably very small. There is such a diversity of life just on the Earth that I believe that any life we do find will look completely different.

If you are interested in reading more about the possibilities of life, Carl Sagan (he was an astronomer at Cornell) has written about this, I think in the book "Pale Blue Dot". There is also a theory about the chances of life forming on other planets, called the Drake Equation. This is an equation which an astronomer named Frank Drake (he's a professor at UC-Santa Cruz) came up with to predict the number of civilizations there might be in the Milky Way trying to communicate with us. There are a number of good web pages about this if you search on, but here is a quick, simple explanation of it:

Matthew Knight

6. Asked about the recent discoveries that have been made on Mars and was wondering about the possibility that there was life there in the past, or even at the present time.

In the past ten years, there have been some fascinating discoveries on Mars indicating that Mars had liquid water in the past, and may have pockets of it even today.  The reason that this is fascinating is that all of the life forms that we know about require liquid water to develop.  If water was never present on Mars in liquid form, then it is not likely that life could ever have developed there. If liquid water was present, however, then life could have formed, and may still be there today.

Recent satellite pictures of the surface of Mars have shown riverbeds, flood plains and drainage basins.  From these pictures, it is not clear whether those features represent a time when liquid water was common, or if they were produced by short episodes where sub-surface ices were melted (by a meteor hitting the surface, for example).  If they were short-lived events, then life would not have had time to form so the key is to determine whether the water was present long enough for life to form.

Early this year, the Mars rovers, Spirit and Opportunity, found evidence confirming that water was present in liquid form for extended periods of time.  This means that water may have been present long enough for life to have formed!  The fact that water was present, however is not proof that life did form.  The rovers were designed to look for evidence of water, not life, and even though they have found evidence of water, they have found no direct evidence that any type of life has formed there.

In the late 1970s, two other Mars Landers, Viking I and II had experiments that were designed to look for life.  The results they found showed that there was no life.  However, the Viking spacecraft did not land in sites that were particularly good for life to form (they were not places where water was likely to stay).  In addition, the same tests used on the Viking spacecraft were done in many places on Earth, and in some of those sites, the results also claimed there was no life on Earth.  So the fact that they didn't find life with Viking, is again not conclusive proof that life never existed.

Finally, there may be some proof that life existed on Mars in the past.  Millions of years ago, a meteorite struck Mars with enough force that it blew pieces of the surface into space. Some of those fragments slowly made their way to the Earth, and landed in Antarctica as meteorites.  They were found a few years ago, and microscopic analysis revealed small fossils that may have been one-celled life forms.  This is a controversial result, because these cells are not like anything known on Earth, so it is not clear that they were really ever alive.

So, is there life on Mars?  Was there ever life on Mars?  The current evidence suggests that there is not now nor have there ever been any large life forms.  No Martians that could have come to Earth in their spaceships, like you see in science fiction movies.

However, there could still be tiny life forms, such as bacteria, that we just haven't found yet.  Now that we know there was water on Mars, future missions to Mars will be designed specifically to look for this type of life, or for evidence that it existed in the past.  If evidence of this type of life is found, then this would be a fantastic result, because it means that life is present elsewhere besides Earth. If it exists on Mars, it could also exist in other places (Jupiter's moon Europa might have liquid water, too, for example).  And if life can exists elsewhere in our solar system, then it can exist in other solar systems, and we may not be alone.

Cole Miller

7. We were recently discussing carbon-based life forms and the chemistry involved with ammonia, methane, water vapor, etc., the famous Urey-Miller experiment of the 50's, and the connection between carbon and organics.  When you look at the periodic table, you can see that silicon is in the same family as carbon (as well as Ge, Sn, Pb, etc.).   Wouldn't research on how other elements could support life in different environments give us an idea of the possibility of life elsewhere?

That *is* a good question.  I've asked similar things about silicon, and have been told that although silicon is indeed a candidate as a backbone of life (maybe rocky life!), it is slightly worse than carbon in several important ways.  For example, carbon has strong bonds with itself and with other atoms, giving it tremendous flexibility in making bonds.  Therefore, you can have long molecules that have carbon chain backbones.  Apparently, silicon chains of the same type would be much more fragile.  However, I, too, would like to know more about this, in particular whether there are certain temperatures, pressures, or chemical environments in which silicon can be a good base for life. Maybe some day you can find the answer and tell me!

Cole Miller

8. Suppose we (meaning Earth and organisms living here) are the only source of life in the universe, and looking at the way we treat our world it shall not last too long, do you think it is possible to spread our living capabilities to other planets/moons to be able to preserve life longer? Or how long do you think it will be until we have a way of creating a artificial atmosphere that could sustain life on other places?

Presently, we do not have the technology to colonize another planet.  The first glaring obstacle is the means of transport.  I believe it takes us about 6 months just to get to Mars.  The closest star is 4.3 light years away, meaning it takes light 4.3 years to get from that star, Alpha Centauri, to us. Since our space-crafts travel much slower than light, it would take an excruciating amount of time to travel to another planet orbiting a different star.  Of course, before we decide to colonize another planet, we would need to put a lot of effort studying that particular planet.

Colonizing Mars may become a possibility, but we still need to learn a lot about it.  NASA certainly has a keen interest in Mars, because Mars has many similarities to Earth.  There is evidence that water may have been prevalent on Mars, which is a necessity for sustaining life; but again, we are still in the process of learning about it.

Putting a time frame on how long it would take to do certain things is quite difficult.  It looks unlikely that we will colonize another planet in the next couple hundred years.  However, given NASA's interest in Mars, we (you and I) may be around when a human steps foot on Mars.  That in itself will be quite an accomplishment.

It is true that our natural resources are being consumed at an alarmingly rapid pace, but there is hope that we can change this.  Many people and organizations are working hard for that cause.  Scientists are also researching energy sources that are not so harmful to our environment. So, if we take care of our own planet, then having to colonize another planet just to sustain life will not be necessary.

Rahul Shetty

9. Does extraterrestrial life exist?

The question of whether or not extraterrestrial life exists is fascinating.  We have only one example of a planet where this has happened--our Earth. To address the question scientifically, we must base our reasoning on experiences here.  From your questions, I will assume that you are interested in intelligent life that has developed a civilization. This is what we know based on studies of our own civilization and life here on Earth.

A. Life on Earth involves complex chemistry (respiration, reproduction, photosynthesis in plants, and metabolism) that centers around the carbon atom. The large molecules of life (DNA, RNA, proteins, etc) use carbon as the backbone with liquid water to bring in materials and expel waste products. Because of the special chemical properties of carbon and water, this combination makes sense and if life does exist elsewhere, odds are that it will have similar chemistry.

B. Planets that have rocky surfaces and that are located near enough to their stars so that liquid water exists on the surface would be most like Earth, and astronomers suspect that this kind of planet is fairly abundant.  The odds that the planet will have conditions that support liquid water are more uncertain.  There are a lot of variables.

C. The star should have a long stable life.  Stars like the Sun (about 10% of the 400 billion stars in the Milky Way Galaxy) typically last 10 billion years before they change significantly.  It took a very long time to go from microbes to an intelligent civilization (us) here, about 3.5 billion years.  If this is the usual path to a civilization, not all stars are ideal. Some stars change rapidly in less than a billion years and some stars are just too cool.

D. Changes in a planet's environment (tropical ages, ice ages, a climate altering impact like the one that triggered the mass extinction that included the dinosaurs) can cause the extinction of some species and make room for new species to fill their niches.  Even if you started with a planet identical to Earth, it is unlikely that you would end up with "human beings".  You may get an intelligent life form based on carbon chemistry, but they wouldn't look exactly like us. Intelligence seems to provide an evolutionary advantage, so natural selection favors it.  The same should apply elsewhere. 5. Once a civilization develops a technology (usually defined as the ability to use radio telescopes to send or receive signals over interstellar distances), again if we are typical, the ability for self-destruction is also there.  Along with the development of radio telescopes at the end of World War II came the ability to build nuclear bombs, chemicals that can alter the environment, pollution, etc.

I would say that it looks good for extraterrestrial life based on these factors: knowing that the chemicals of life are fairly abundant in the galaxy, that it looks good for earth-like planets around other stars that are similar to the Sun, that sometimes clearing out many species makes room for new ones to evolve (age of mammals after age of reptiles), that intelligent species will see the advantage of working together in a civilized society.  The development of science and technology may follow since in our species intelligence seemed to promote curiosity about the world/universe.  So these are my thoughts and opinions of extraterrestrial civilizations.  There may not be many of them but I don't think that we are alone.

The best place to get information about SETI is at Another search project is serendip out of UC Berkeley a http:/ So far no signals detected that have repeated.  There is a plan in case a signal if detected and confirmed.

Grace Deming

10. Scientifically, do you think that something could evolve, whether animal or plant, to live on a gaseous planet that we can not yet see? How would their atmosphere affect their evolution and adaptation to their environment? We realize everything evolves differently according to their circumstances, so what would other life, or intelligences even, look like?

Well, I'm not a chemist and I don't know that much about primordial biology, but let's think this through and see where it gets us.

Now, as I understand it, the theory of evolution postulates that life started because of chemistry.  In the primordial oceans of Earth there were all kinds of things dissolved in water, very little oxygen, and very different temperatures and lighting conditions from what we have today.  In this environment, proteins and nucleic acids formed and, over millions of years, eventually managed to combine into a form of primitive RNA, and life began.

So, if you have a planet with millions of years to spend under the same conditions of temperature, chemical content and moistness as the early Earth, then it seems to me like there is a chance that life might start there as well.  And if it does start the same way as it did on Earth, on a planet very much like Earth, then it wouldn't be too surprising if the creatures that evolved, including the intelligent ones, ended up looking a lot like the ones on Earth.  There will be differences, surely - look at the differences just between animals from different continents on our own planet - but on the whole you can probably expect to find buggy things, fishy things, lizardy things, and furry things that walk on two or four legs.  Maybe if evolutionary pressures are right your intelligent creatures might end up being, say, scaly instead of hairy.

But if I understand the chemistry correctly, the processes that might have led to RNA forming on Earth are not easily transferable to non-Earth conditions.  Too cold, for example, and the molecules won't have the energy needed to react and combine.  Too warm, and the proteins and nucleic acids will break down before they can combine with each other.  Too much water and they won't be close enough to each other to react.  Too dry, and they won't be able to slide around and mix with each other enough to make the bigger structures needed.  So it's a pretty delicate balance.  Consequently, if we're talking about DNA-based intelligent life, I'd have to say it's probably chemically impossible on planets that don't look a lot like Earth.

What I don't know, and I don't know if anybody DOES know, is whether it's possible to have life based on chemicals other than proteins and nucleic acids.  Basically, what you need are fairly complex molecules that can combine into more complicated structures, and you need those structures to be able to copy themselves reliably, but still allow for the occasional mistake (i.e., a mutation) so that evolution can happen.  Is it possible? That's the sort of question you can't really answer with "no", because there are just about an infinite number of ways of combining various atoms into molecules and compounds, and we can't possible check them all.  Now, if you actually found a life form based on something else, then you could definitely say "yes", but you really can't definitely say "no".

OK, so let's assume that somehow, in some way we don't yet know about, it is possible to form molecules in the atmosphere of a gas planet that could combine and evolve the way proteins and nucleic acids seem to have done on Earth.  So we're assuming that it might be possible to have life evolve on a gas planet.  Evolutionary theory says that the forms that evolve will be in response to environmental pressures.  So, we should be able to make some predictions based on what we know about the environment of a gas planet.

For example, the vast majority of larger animals on Earth have right-left symmetry.  This has the advantage of making it easier to control motion. Things that walk have two legs or four, for example, but not three.  If you've got two legs, your left leg is a mirror-image of your right leg, not a completely different shape; and if you've got four legs you generally have two matched pairs.  It's safe to assume that it would be an evolutionary advantage to be able to control your movement in the atmosphere of a gas planet as well, so we can imagine animals having symmetric pairs of wings, or other external body parts.  It also helps to be able to find and chase your food, so something equivalent to our own five senses would also likely evolve in the higher animals.  But the gas-planet eyes, ears, nose, etc., would evolve to work best in, say, a cloudy, acid-filled, windy environment - so they probably wouldn't look anything like what we're used to with common Earth animals.  It's easy to imagine that intelligent creatures would evolve, since if you're trying to avoid being eaten, it helps to be smarter than whatever is trying to eat you.

Of course, creatures living in a gas planet would have a density based on where in the atmosphere they live.  Denser (and probably larger) animals would live in a lower, denser part of the atmosphere and might be crushed if they go too low, or explode if they go too high.  Without a solid surface they wouldn't need feet, but the larger animals would need some way to catch and maybe hold their prey, and smaller animals would probably need some way to flee.  If you can't use legs to move, you'd need some other way to get around, like maybe sails or wings or bladders that could be inflated and then rapidly deflated to produce puffs of wind.  Hm - maybe the planet would be full of little animals scooting around making whoopee cushion noises!

I remember vividly a television mini-series in the 1970s called "Cosmos". It was written and narrated by the famous astronomer Carl Sagan, now deceased, who was also very interested in the possibility of life on other planets.  In one episode he actually talked about what creatures that evolved on a planet like Jupiter - a gas giant with no solid surface - might look like.  If I remember correctly, he thought about how weather and winds affect the atmosphere of Jupiter, and how smaller creatures (like plankton and algae in our oceans) might live in specific belts where conditions where right for them.  He finally presented an artist's conception of what the larger creatures might look like, and if I remember correctly they looked kind of like a cross between a jellyfish and a hot-air balloon.

The book based on the "Cosmos" series is still around, although it might not be in print at the moment.  Check your library and see if you can find it.  The sections on evolution and how extraterrestrial life might evolve are fascinating.  Even better, your library might have a copy of the series on DVD.  I highly recommend it.

The study of the various factors affecting the potential evolution of life on other planets is called "Astrobiology", and it is still relatively new, as sciences go.  Here's a URL you might find interesting:

This will take you to the home page of the SETI Institute (SETI stands for "Search for Extra-Terrestrial Intelligence").  The scientists who work at this institute are seriously interested in the possibility of life on other planets, what it might look like and how we might be able to discover it.

Ultimately, the only answer I can really give you to your questions is, "We don't know."  But as you can see, by applying what we do know, recognizing the patterns, and then using logic and imagination, we can take some good guesses.  As long as we keep our minds open, there's a good chance that if we do someday come across life on another planer, we might actually recognize it.

Anne Raugh

11. If the earth where farther out in space, is it hot enough in the center to heat itself?  I do know that some life live in the volcanic vents, so could at least that life be sustained? If not, how hot would our planet have to be to heat itself from the center out?

That's an interesting question.  Most of the heat on the Earth comes from the Sun, and in particular all surface water would be frozen solid if the Earth were way out in space.  Life as we know it depends on liquid water, so that wouldn't be so good.  However, it might be that much lower down, where heat from radioactive decay (of heavy elements such as uranium in Earth's interior) leads to a warmer temperature, some liquid water would exist.  Maybe then there would be bacteria.  Also, perhaps there would be a form of life that doesn't depend on liquid water.  In any case, parts of the Earth's interior would still be molten, so it wouldn't be a rigid rock.

A planet as large as Jupiter actually produces most of its own energy (by slow settling and gravitational contraction) rather than getting it from the Sun, so although it would be colder in deep space it might be warm enough that in its deep atmosphere life could exist. Either way, though, based on what little we know about the conditions for life, it seems likely that any such life would be microbes and not big things with a chance at intelligence.

Cole Miller

12. Can there be extraterrestrial life and where would it be?

Questions about extraterrestrial life are fascinating but difficult to answer.  That's because we only know of one place (Earth!) where life exists, meaning that it is difficult to say for sure what is necessary for life.  We can say that, here on Earth, bacterial life appears to be in every nook and cranny we can imagine (and many we can't) where two things hold: (1) there is a source of energy to tap, and (2) liquid water can exist sometimes.  There are, for example, flourishing ecosystems near hot sulfur vents at the bottom of the ocean, and microbes that exist two miles beneath the surface of the Earth in tiny cracks in rocks.  Pretty amazing, and a lesson to not limit what life can do!

With this in mind, the best we can do is determine what appears to be useful for life here, then ask where else might have similar conditions. Liquid water seems to be really useful for all life on Earth, so where else in our solar system might have liquid water?  To answer the question, there is evidence that Mars has liquid water below the surface in some places, so Mars is a reasonable prospect for life.  Jupiter's moon Europa is covered in ice, but the patterns of cracks suggest that there is liquid water maybe several miles down.  Therefore, that might be a place where life could exist.  In both cases, though (and in others such as some additional moons of Jupiter), it is very likely that the life would be bacteria or at most very small non-intelligent things, so communicating with that life wouldn't be an option.  Don't get me wrong, though: detecting even bacteria that are definitely not of Earth origin would be really exciting!

In terms of life on planets around other stars, people have discussed ways of getting some (admittedly indirect) evidence for life.  For example, suppose that at some point we are able to get really good spectra of the atmospheres of extrasolar planets, and we found that there is a lot of molecular oxygen (that is, O2, which we breathe).  Believe it or not, that might suggest that there is life present.  That's because in its natural state, oxygen combines with other stuff pretty easily (as you have experienced when you see things that rust or burn!).  In our atmosphere, oxygen is a byproduct of photosynthesis, which is performed by living things (plants).  Of course, unless we detect radio signals from an intelligent source we won't be absolutely sure, so at this point there is no direct evidence of life anywhere but Earth.

Applying some physics and astrophysics:  On land, there are various things that limit the size of animals, but one factor is simply gravity.  All else being equal, a more massive planet has stronger gravity than a less massive planet. Really large things need thicker legs to support them, and it becomes more difficult to have good blood circulation.  This might mean that a more massive planet would have stronger limits on the size of land animals. In the ocean that shouldn't be a problem, so I don't know if there are fundamental limits there.  From the astrophysics standpoint we know that more massive stars have shorter lives.  That would tend to mean that there is less time for life to evolve, but since we don't have a firm sense of the necessary time for evolution, that might or might not be a big problem.  Therefore, the answer is that I don't know (and I doubt anyone could say for sure), but I would guess that the size of the planet would be important, and that really massive stars (that live only a short time) might not have life at all.

Cole Miller

13. If our galaxy was created by a big explosion, and since that happened, do you think that other galaxies could have had the exact same thing? And does that mean that other galaxies can support life? If so, do you think that there is intelligent life? Thank you so much for your time.

Questions about life are fascinating but at the same time frustrating, because we know so little.  The only place in the universe where we know there is life is Earth.  Therefore, when we consider what might be required for life or how common it is, it's mostly guesswork with a little bit of fact thrown in.

With that in mind, I think the odds are pretty good that life exists elsewhere in some form.  Bacteria seem to be practically everywhere on Earth that has liquid water (including near sulfur vents at the bottom of the ocean and in small cracks in rock!), so with billions of stars with planets (probably) in a galaxy I think it is really likely that some have at least bacteria.  Intelligent life is tougher to speculate about.  Human beings are the only species in this history of Earth to develop technology.  Does that mean that a planet that produces life will always develop technology in a few billion years?  Did we get really lucky on Earth?  Maybe it takes some spectacular accident to develop complex life from single-celled life, and we're the only ones. Tough to say.  My guess, though, is that intelligent life does exist in other galaxies.  We might not find out for sure in our lifetimes, though.

Cole Miller

14. I am really interested in evolution and how it happens. Why do we assume that other beings must be based on carbon?

Good question. Turns out that carbon has some unique properties when it comes to chemistry in environments like those on Earth. I'm not a chemist, so I'm going to refer you to an answer from Dr. Miller to a similar question. See number 7 on this page:

Part of this has to do with the fact that carbon has four electrons in its outer shell available for bonding with other atoms - and therefore the possibility of creating many different types of molecules, including long chains based on a series of carbon atoms linked together. Carbon is also the simplest atom with that structure, and there is a fair amount of it around. Silicon also has four electrons available for bonding, but it is a bigger and heavier atom itself and its outer electrons are less tightly bound to the nucleus than those of carbon, which affects its chemistry. It's also a little less common than carbon.

Other atoms with four free electrons in the outer shell are metals, and metals react VERY differently when it comes to creating compounds.

Silicon is the next-best candidate if you want life forms that might evolve to look something like those on Earth, because it has an atomic structure very similar to carbon. But it does react differently from carbon in the same situation - so you can't just replace carbon atoms with silicon atoms and expect EXACTLY the same sort of life to evolve. That's why I would be very surprised if silicon-based life forms and carbon-based life forms could share the same planet. But I wouldn't be so surprised if it turns out there is an environment where only silicon-based life forms could survive.

(See also #10 in the URL above for an answer to a previous question about life on a gaseous planet.)

I believe life elsewhere is possible, and I think the possibility of finding primitive life forms in, for example, the sub-surface oceans of some of the outer moons is very exciting. I don't expect to find them swimming around with iPods in whatever they have for ears - because I think if there were intelligent, technological life forms like us in our own solar system we'd have noticed something by now. But I'd be happy to be wrong about that. I know *I* can be pretty dense sometimes.

Intelligent life elsewhere someplace in the universe? Sure, why not. It happened once, so we know it's possible in at least one set of circumstances, and it's a REALLY big universe. But would we recognize them from this far away and be able to communicate? If they're very different from us we might be pointing our telescopes right at them and just not seeing them. And they might be doing exactly the same thing and missing us.

And we'll probably keep on missing each other until at least one of us becomes clever enough to figure out how to detect intelligence from hundreds of light years away.

Thanks for your questions!

-Anne Raugh

15. From what you know, where would you say the most likely place for life in the universe would be?

Well, the most likely place I would look is here on Earth. I think I'd have a pretty good chance of finding life, including intelligent life, here.

But I'm guessing that you are looking for an answer regarding extraterrestrial life. This is a more complicated issue, because we haven't found life anywhere else. At least not yet. Life as we know it (based on carbon, etc) requires certain conditions to form, and it is these conditions that suggest where we should look.

It is thought that the single most important requirement that is needed to form life is an environment that allows water to exist in a liquid state for a long period of time (billions of years). Liquid water not only provides some of the elements needed for life (hydrogen and oxygen) but it also acts as a factory, where other materials can flow around and meet other materials where they can combine into the more complex molecules that form life as we know it. The mechanisms for this process can take a long time to occur, so the liquid water must be available for a long time. Given these constraints, we can narrow our search for life to very specific areas.

Within our solar system, there are only a few places where liquid water can exist (besides the Earth). These are currently being explored to determine if life is currently present, or has ever been present in these locations. Recent explorations of Mars have shown a lot of evidence that there was once liquid water on the surface, though at the present time there is none. Some scientists have suggested that there may be liquid water below the surface, and simple life forms, like bacteria, could exist underground. There are plans being put together to look for this sub-surface water, and to explore the potential for life.

A second place that life could exist is on Europa, one of the moons of Jupiter. This is a large moon with an icy surface, but the cracks in the ice form patterns that strongly suggest there is liquid water under the surface. And if there is liquid water, there could also be life. Exploration of this moon promises to be a hot topic in the future.

Outside our solar system, we are more limited in how we can search for life. First we need to find potential planets where water may exist. There are several searches being done for planets that revolve around other stars. Because of the way that these searches work, most of the planets that have been found so far are very large (the size of Jupiter or bigger), and very close to the star they orbit. Only in the past couple years have smaller planets been found. In the next decade or so, many more Earth-sized planets should be detected, and then we can really start to look for potential places where life may have formed. Eventually, as more planets are found, we can start to focus on properties that favor life: smaller, rocky planets (as opposed to gas giants like Jupiter), that are orbiting around long lived stars (so life has time to form) at distances where water can exist in liquid form (not too hot and not too cold).

Since even the nearest stars are many light years away, we can't directly observe life itself on these planets, so we need to look for indirect evidence that might prove that there is life. This is done by looking at the spectra of the planet, to see if they have atmospheres, and if so, then do those atmospheres have certain properties that indicate there is life? If the atmosphere contains water, then it is possible that liquid water could also exist, and conditions may be favorable for life to form. This is still not proof that life exists, however. (Just a note: last week, there was an announcement that an extrasolar planet (HD209458b) has been found to have water in its atmosphere, but it orbits very close to its star, and so probably doesn't have liquid water.)

To prove there is life, we need to find some kind of signature that specifically shows life is present. One possible indicator is the presence of large amounts of oxygen in gaseous form. Oxygen likes to combine with other molecules, and over time, it will all combine to form gases like carbon dioxide, leaving little gaseous oxygen. Only if there is new oxygen being constantly formed will there be gaseous oxygen in the atmosphere, and life is a very good way to make new oxygen. (For comparison, both Mars and Venus have atmospheres dominated by carbon dioxide, while the Earth's atmosphere is 21% oxygen. Earth is the only one that contains a lot of life.) So if we look for oxygen around the extra-solar planets, this could be our first indication of whether they have life or not.

Unfortunately, finding the signature of life is only part of the problem. The presence of oxygen doesn't give any way to differentiate between whether that life is in the form of algae or highly intelligent beings. This is a step-by-step process, however. First we need to find life in some form, then we can start to figure out what form it takes.

Another search includes the SETI project, which scans the sky for radio signals that are produced by intelligent life. They hope to detect either direct communications from other civilizations, or just the general background signal that might come from things like our radio or television broadcasts. This is an exciting project, because it does involve intelligent life, rather than just life in general.

Finally, there is always the question about whether or not there may be different types of life that could be significantly different from "life as we know it". One suggestion is that silicon-based life could have evolved separately from carbon-based life. Currently there is no evidence of this, and so we don't know what to look for to identify these other types of life form. Not knowing what to look for makes it difficult to look for them.

The universe is a big place, and it's likely that there is a lot of life around. It's just a matter of improving our techniques until we find it.

Tony Farnham

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