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Ask an Astrophysicist: Astrobiology

Ask an Astrophysicist

Library of Past Questions


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Library of Past Questions and Answers


Can you tell me more about those new planets that were found? (Submitted October 24, 1996)

The Question

Can you tell me more about those two new planets that were discovered?

The Answer

To answer any question you have about planets which have recently been discovered orbiting other stars, there are a number of places to look.

You might want to look at the article by David Black in the August 1996 issue of Sky and Telescope. We think David does a very nice job of describing what we know now and what might be learned in the next few years.

There are also a number of good web sites. Jean Schneider, of the Observatoire de Paris, runs the Extrasolar Planets Encyclopedia. There are a number of pages run by extrasolar planet research groups themselves. Marcy and Butler work out of San Francisco State University, and Alex Wolszczan, the first to discover an extrasolar planet, works out of Penn State.

Question ID: 961024

How common are planetary systems around other stars? (Submitted September 22, 1997)

The Question

How common are planetary systems around other stars?

The Answer

The evidence is mounting that planets are quite common around other stars. Because the mass of any planet around it's parent star is much less than the star itself, it is difficult for us to observe the effects of the planets from Earth, and it really isn't possible right now to make an intelligent estimate of the percentage of other stars that have planets. However, a number of different observational studies have results that when combined imply that planets are the rule and not the exception. For example, theorists believe that planets form from a disk of material circling a star when it is young. Observations of young stars with the Hubble Space Telescope and other instruments have directly imaged such circumstellar material. Very young stars are also found to show evidence of jets coming out of the poles---jets are a very strong indicator of a disk structure.

Researchers who study the precise timing of pulsars have found that some pulsars show a wobble in the period of the pulsar. That is, instead of the pulsar always having the same period, sometimes it is slightly faster, and other times slightly slower. This is strong evidence for orbital motion of the pulsar about the center of mass of a system. From careful analysis of the pulsars' period changes, orbits have been deduced that suggest planets circle the pulsars.

Within the last two years or so, a number of research collaborations, including ones in the US, have found compelling evidence for Jupiter-sized planets around relatively nearby Sun-like stars. These systems are mush more similar to the Sun and our planets than are the systems around pulsars. These teams use very precise measurements of the stars' radial velocity, that is, the speed at which it is moving toward or away from us. If the star has planets a very small wobble will be seen in the radial velocity. They show that many stars may host Jupiter-like planets, some in very close, or very eccentric orbits. The hunt is on for more planets around Sun-like stars, now that the technique has proven fruitful!

So, while I can not give you an exact numerical answer to your question, I hope I have given you an idea of why astronomers have recently begun to suspect planets around many stars.

For more information, check out

There is a section on extrasolar planets in WebStars with links to sites elsewhere on the subject. This, and many other topics, are discussed in WebStars at

In particular, there is information on pulsar planets at


Padi Boyd
for the Ask an Astrophysicist

Question ID: 970922f

Can you tell me about the new solar system that has been found? (Submitted April 19, 1999)

The Question

I am in the 7th grade, and have a great interest in astronomy. A couple of days ago, I read an article in the newspaper about a new solar system that has been found, with multiple planets orbiting a star. Could you tell me where to find more information on this?

The Answer

The star in question is Upsilon Andromedae. The discovery of 2nd and 3rd planets is very new, in fact this hasn't been published yet in a proper form. (The discovery of the first planet was announced in 1997).

The group who announced this discovery has a web site on it:

A good site for extrasolar planets in general is:

Hope this helps,

Koji Mukai
for "Ask an Astrophysicist"

Question ID: new_planets

How are solar systems created? (Submitted February 15, 1998)

The Question

How is a solar system such as ours created?

The Answer

A solar system is created when a rotating cloud of gas and dust in space start to coalesce - they are pulled together and towards the center of the gas/dust cloud by their gravitational attraction to each other. As they condense, the particles collide faster and more often, which causes the gas and dust to heat up. The gas and dust at the center collapses to form the central star of the solar system; the heat generated by the colliding particles starts nuclear fusion in its core. If there was enough angular momentum in the system at the very beginning, then not all of the dust and gas will go into the central star - the rest will remain in a flattened disk around the star. The planets form from this disk of rotating material as it clumps together because of gravity.

There are a number of places on the web where you can go to for a more detailed description of the process, and even some numerical simulations of this.

J. Allie Hajian
for Ask an Astrophysicist

Question ID: 980215a

Can you tell me the differences between exoplanets and brown dwarfs? (Submitted October 23, 1997)

The Question

Can you tell me the differences between exoplanets and brown dwarfs please. Also I could not find any book regarding this. Is it because brown dwarfs and exoplanets are new to us?

The Answer

This is somewhat outside our field of expertise (which is X-ray, gamma-ray and cosmic ray astrophysics), but I've managed to find a definition in the 'EXOPLANETS' pages at:

The difference is in how they formed: brown dwarfs formed through the collapse of a molecular cloud, planets formed around a protostar through accretion of planetesimals and gas.

I think the subject of (theoretical studies of, and searches for) brown dwarfs and exoplanets have been with us for a long time. However, until recently, we had had very little data --- it is only within the last few years that many important discoveries have been made. So I think you are basically correct as to why you cannot find books on these subjects.

Best wishes,

Koji Mukai
for Ask an Astrophysicist

Question ID: 971023e

    Life in Space

What are the remote indicators of life on Earth from space? (Submitted April 23, 1998)

The Question

Imagine you were an alien roaming around in Space and you were interested in finding a planet that has life. What is the evidence that Earth has human life? What are the unambiguous indications of life on Earth?

The Answer

There are a combination of features of the Earth that you can detect from space that would be very hard to explain without invoking the presence of life. These are: the abundance of molecular oxygen; the presence of chlorophyll, the pigment used by plants in combining carbon dioxide and water to form sugar; and a trace amount of methane (which is hard to maintain with all that oxygen around).

It so happens that the Galileo spacecraft (see, for example ) reached its ultimate destination, Jupiter, via a circuitous route (for technical reasons), including two flybys of Earth. NASA attempted, and succeeded, in detecting the abovementioned signs of life on Earth during one of them.

(Also if you are talking about intelligent life forms, radio and TV broadcast signals would be a conclusive evidence.)

Of course, there is no guarantee that every planet with living beings will look like the Earth (it's hard to be sure when you have only one example!). In fact, Europa, a moon of Jupiter, seems to have a liquid ocean underneath its icy crust, and some scientists speculate that there could be life on Europa. In this case, the ice on the surface makes it difficult to prove or disprove this idea from a distance. (The JPL Galileo pages above have a lot of information about Europa.)

Best wishes,

Koji Mukai
for Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See for help on other astronomy Q&A services.

Question ID: 980423a

Has evidence been found for life on Mars? (Submitted December 23, 1996)

The Question

Has evidence been found for life on Mars?

The Answer

We are a center for research in high-energy astrophysics and have no institutional experience in the search for life in Mars (though back in 1976 I was a student intern with the Viking Project at JPL!). There is a lot of very good material available on the Internet concerning the recent discoveries within Martian meteorites of possible evidence for past life on Mars. A good starting point is:

  • which has links for each Martian meteorite, as well as to the various current Mars missions and many news stories concerning the recent discoveries.

Although each piece of evidence (morphological, mineralogical, chemical and isotopic) presented so far can be explained by non-biological as well as biological processes, their presence in close proximity to each other, and in two quite different Martian meteorites (the old ALH84001 and the much younger EETA79001) makes further investigation a clear priority. The two NASA spacecraft now on route to Mars - Mars Pathfinder and Mars Global Surveyor - will prepare the way for future sample return missions. In the meantime, a lot of work can be done using the Martian meteorites already available.

As of October, 1998, the evidence against microfossils in AH84001 is now even stronger, with the discovery that some lunar meteorites have objects similar to the 'microfossils' found in the Martian meteorite.

Details can be found off the JPL Mars Meteorite Home Page at:

Best wishes,
Paul Butterworth
Imagine the Universe!

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Question ID: 961223b

What would an alien be like on Europa? (Submitted October 07, 1996)

The Question

What would alien life be like on Europa? I understand that the icy moon does have an atmosphere, though a very thin one, and that it contains oxygen obtained differently than on earth. Could this atmosphere support life? Could the dramatic temperature changes affect life (if there is) when Europa is blocked from the Sun because of Jupiter?

The Answer

As I am sure you already know, Europa is one of Jupiter's moons that Galileo discovered using a small (and very crude by modern standards) telescope.

Remember, there is one place in the solar system that we know life exists now -- Earth. So, you might want to think about comparing Europa to Earth as much as possible.

Europa's atmosphere does contain oxygen but it is extremely thin so it is hard to see how it could support life. However, Europa might have oceans under its icy crust. These would be kept liquid by heat from tidal friction. These hypothetical oceans would be more hospitable to life than the surface. Since they would be below a layer of ice and their main heat source would be tidal friction, having the Sun blocked by Jupiter wouldn't make much difference to them.

Some basic questions you should ask about Europa:

  • Is there an atmosphere? If so, what is it made of?
  • What is the surface gravity? (on Earth it is 1"g" = 9.8 m/s/s = 32 ft/s/s )
  • What is the temperature? How different is it from day to night?
  • How long are the days?
  • Does Jupiter block the Sun often?
  • Is there volcanic activity? If so, how does this affect it?
  • Are there more questions I should be asking?
Some basic questions you should ask about your creature:
  • Where does it get its energy? (We get ours from eating and breathing and then "burning" the food, to be very simplistic.)
  • How does it keep its body temperature about right? What is "about right" and why?
  • How does it move around -- or does it not have to? How does it reproduce?

For more information about Europa take a look at

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See for help on other astronomy Q&A services.

Question ID: 961007

Is there any other form of life out there? (Submitted November 19, 1996)

The Question

My question is what is really out there? Is there any form of life out there?

The Answer

Your question is a good one and, since I am not a real expert in any of these subjects, I think the best way for you to learn more is if you look at some of the web pages which have answers. I can say that many scientists think it is possible that there is life elsewhere in the Universe, but that finding it may prove difficult. First of all, we don't know exactly what form life on other planets outside our solar system might take, so it is difficult to know how to search for it. Second, the universe is a big place, and radio signals (and space vehicles) travel too slowly to allow us to find life that is very far away. We do know something about the conditions on Mars, the planet in our solar system which is most likely to have life, and it turns out to be similar to the conditions in Antarctica (except for the lack of oxygen). So we can guess what forms of life might be able to survive there, and there are projects under way that will someday send machines to Mars which can look for these kinds of life. There is more information about the exploration of Mars, for example in

Also, the SETI ("Search for ExtraTerrestrial Intelligence") institute has some pages which may be useful:

I hope this is helpful to you!

Tim Kallman
(for the Ask an Astrophysicist team)

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Question ID: 961119b

What are the chances of life existing outside our solar system? (Submitted September 24, 1997)

The Question

What are the chances of life existing outside our solar system?

The Answer

This is a question that astronomers first started to quantify in the early 1960s. In 1961, a radio astronomer named Frank Drake developed an equation to stimulate discussion of the Search for Extraterrestrial Intelligence (SETI). This equation, which is now called the Drake equation, states that the number of communicating civilizations in our galaxy likely depends on a number of factors which must combine to yield a habitable planet where life has the chance develop to a certain level of technological know-how. These factors include the rate of formation of stars like the Sun, the fraction of those with planets, the fraction of Earth-like planets, the fraction of such planets where life develops, the fraction of those where the life becomes intelligent, the fraction of intelligent species who can communicate in a way we would detect, and the lifetime of the communicating civilizations. As you may imagine, there is a lot of debate about reasonable values for most of these factors. As we learn more about the likelihood of planets around other stars, we are able to better estimate one of these parameters. For the other parameters, the estimates vary widely. Frank Drake's own current estimate puts the number of communicating civilizations in the galaxy at 10,000.

You can find out more about the Drake Equation from

Padi Boyd
for the Ask an Astrophysicist

Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See for help on other astronomy Q&A services.

Question ID: 970924

Where can I find information on extraterrestrial life? (Submitted March 17, 1997)

The Question

I'm an undergraduate student in Physics and I would like to have some information on extra-terrestrial life for my astrophysics project.

It would also be a help if you could propose some web sites.

The Answer

There has been quite a bit of work on extraterrestrial life, both from a theoretical point of view and also based on the known properties of the planets in our solar system, notably mars. A place to start is the SETI institute.

We are interested primarily in X-rays, gamma-rays, and cosmic rays in astronomy, and so can claim no particular expertise in this area. I hope that these references are some help.

Tim Kallman
for the Ask an Astrophysicist team.

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Question ID: 970317b

Could life based on an element other than carbon exist? (Submitted February 21, 1998)

The Question

I am a student currently in the 10th grade. I have been curious to whether life could exist on Europa (or any other unexplored space body) in different forms. For example, since Earth is shaped around the element, carbon, could life exist revolving around a different element?

The Answer

Scientists have occasionally speculated that life could be based on an element other than carbon. Silicon, being the lightest element with an electronic structure analogous to that of carbon (having a half-filled outer shell with 4 unpaired electrons), is the most likely candidate mentioned. However, carbon's tendency to form the long chains and rings that form the basis for organic compounds that at some level of complexity begin to self-replicate is unique. Also, because older stars naturally produce carbon, along with nitrogen and oxygen (its neighbors on the periodic table), it is relatively abundant in the universe. Many astrophysicists who study the spectra of stars believe that complex chains and even rings of carbon appear in such unlikely places as stellar envelopes (e.g., in the form of PAHs, polycyclic aromatic hydrocarbons). When such compounds reach cooler regions of space where they can bond with readily available hydrogen, organic compounds as we know them are naturally formed.

Although other elements may form complex, covalently bonded structures, none has the rich molecular variety of carbon. It is the chemistry of carbon that allows us to consider the possibility of life "as we know it" in other parts of the Galaxy and the Universe beyond. We do not know whether Earth-like conditions exist elsewhere; but if they do, it is highly likely that life forms, if they exist, will be based on carbon.

One more point: The organic types of structures appearing in stellar envelopes are very hot and probably stripped of hydrogen, so that they are not themselves alive; it is only when carried off to a more hospitable environment, such as a much cooler planet 100 million miles away or so, that the kind of chemistry required by life becomes possible on a scale large enough to allow for stable development and replication. Water is also a factor, causing the hydrophobic proteins to clump together at all, and serving as a medium a conduit for new material, protection from temperature changes and harmful stellar radiation, etc.

Mark Kowitt and Damian Audley
for Ask an Astrophysicist.

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Question ID: 980221b


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