Support Provided ByLearn More
Space + FlightSpace & Flight

Alien Earths: Expert Q&A

On July 13, 2009, astrophysicist Geoff Marcy of U.C. Berkeley answered questions about the hunt for planets and life beyond our solar system.


Receive emails about upcoming NOVA programs and related content, as well as featured reporting about current events through a science lens.

Q: Have there been any recent refinements to the Drake Equation? If so, what are they? Also, do we have a reliable estimate of the number of stars in our galaxy? I've heard anywhere from 100 billion to 400 billion. Ross Meyer, Monument, Colorado

Support Provided ByLearn More
Q: What is the current estimate of the probability of Earth-like planets in the universe? Anonymous

[Editor's Note: The Drake Equation, named after its creator, radio astronomer Frank Drake, is a mathematical equation for exploring the probability that intelligent life exists beyond Earth.]

Geoff Marcy: The Drake Equation is alive and well! And now we have news. The discovery of jupiters, saturns, and neptunes, along with protoplanetary disks around young stars, provides convincing evidence that Earth-sized planets are probably common. The Kepler mission will tell us the exact frequency of such planets.

Taking all of the evidence in hand, probably 30-50 percent of all stars have Earth-sized planets. Surely about a fourth are in the habitable zone where the water would be liquid. So, probably at least one out of every 10 stars has a habitable Earth-like planet. In our Milky Way Galaxy, with its 200 billions stars, this means that there are 20 billion Earth-like planets in our galaxy alone. Each of these 20 billion planets offers a separate throw of the biological dice in the cosmic chances for life.

Drake's Equation now allows us to fill in one number! The remaining question is how many Earth-like planets spawn technological life. For that, the planet must have both water and continents, because you can't build computers, clarinets, or spacecraft in the ocean. We don't know how many Earth-like planets have just the right environment to support technological life, nor how long that life will survive against the foibles brought by its own technology.

[Editor's Note: You can try out your own calculations on The Drake Equation.]

Q: We've found enough planets to suggest that planets may be the norm after the first stellar metamorphosis, and it is hard to imagine a Jupiter-sized body without moons; therefore, do any of these close-orbiting gas giants exist within the star's continuous habitable zone (CHZ)? Can we determine a star's CHZ? T.M. Hardaway

Marcy: Great question! Yes, many of the Jupiter-sized and Saturn-sized planets that we've discovered orbit smack within the host star's habitable zone. You can easily determine the continuously habitable zone for any nearby star. The star's luminosity (energy emitted per second) determines the distance a planet must reside from its star so that the equilibrium temperature allows water to be in liquid form, between 0 and 100 ºC (32 and 212 ºF). While we don't have direct evidence of moons around any of the exoplanets, surely, as you say, many of the jupiters do have large moons, making them plausible sites for life.

Q: Is it possible that planets are orbiting their stars on a vertical axis rather than horizontal? Meaning from our perspective, are there stars that may have Earth-like planets that we wouldn't see the eclipse of because the planet simply does not pass between the star and our view? Is there any kind of way of detecting those planets being devised? Jeff, New York, New York

Marcy: This is a good point. Most Earth-like planets will not reside in an orbit that is edge-on from our vantage point. Instead, the orbit will be tilted somewhat (and perhaps even "vertical"), so that the Earth-like planet never crosses the face of the star. In such cases, the Kepler mission will miss those planets. Currently we don't have a method of detecting such Earth-like planets. We all hope that NASA will soon build a new spaceborn telescope called "The Space Interferometry Mission" (SIM) that is designed to detect Earth-like planets around nearby stars no matter what sort of orbit they are in. More information about SIM is here:

Q: Are aliens real, and will they try to take over our planet? Thanks. Angela, Charlotte, North Carolina

Marcy: So far, astronomers haven't obtained any convincing evidence of aliens. The hints are all in the form of fuzzy pictures or somebody's story about what they saw. We still await the first firm, 100 percent sure evidence of advanced life forms.

Q: Do you, in your heart, believe that we have been visited? Caroline Who, Fort Myers, Florida

Marcy: I suspect we haven't been visited yet. If we have, they are doing an excellent job of avoiding a clear picture being taken of them. If there really are aliens nearby, they are probably thousands of years advanced in technology compared to us, smart enough to realize that they should leave us humans to develop on our own.

Q: In the past, I have heard of life forms based upon some other substance than water, e.g., methane, silicon, etc. What's the latest thinking on this? Is it merely science fiction? Could it apply to a revised definition of "life?" Jack Harvanek, Chelmsford, Massachusetts

Marcy: There is certainly a chance that some life form exists that is not based on carbon and doesn't need water. A silicon-based life form at high temperatures is one example. The silicon-oxygen chemical bonds might be breakable at temperatures of thousands of degrees, allowing different combinations of molecules to form, similar to the permutations of carbon-based organic chemistry. No one knows for sure if there are such life forms based on something other than carbon and water.

Q: Why can't we calculate the angle of rotation of a star by centering it in the lens, blocking half the telescope, rotating, and measuring redshift and blueshift? Peter Spicer, Redmond, Washington

Marcy: This is a great idea! The reason this technique is difficult is that stars are so far away that they are essentially infinitesimally tiny dots in our telescopes, even with Hubble. So there is no way to split that tiny dot in half. Imagine a grain of sand located a thousand miles away. If it's spinning, there is no way to take a picture of one half or the other because it is too tiny in any telescope.

Q: Can you detect planets around binary star systems? James Coleman, Hegins, Pensylvania

Marcy: Yes! Astronomers have discovered about 20 planets that orbit in a binary star system. In all cases, the planet orbits one star or the other, and the two stars are fairly well separated from each other, at least as far apart as Saturn is from the Sun. That separation allows a planet to orbit close to one star or the other.

Q: Iota Draconis b was discovered in 2002 as the planet that circumnavigates Iota Draconis, an orange giant star located 103 light-years away in the constellation Draco. Can you, or have you looked at the atmosphere of this planet to see if there is a possibility of life? Dr. Karen Frasier-Scott, Houston, Texas

Marcy: This is great question. Unfortunately, there is no way for us to detect the atmosphere of that planet. In fact, we still haven't gathered any light at all from that planet. We have only detected the gravitational reflex motion of the host star. We need a giant telescope in space to take a picture of the planet, well separated from its host star. NASA hopes to build a giant spaceborn telescope called "The Terrestrial Planet Finder" (TPF) that can take pictures of, and analyze the atmospheres of, planets around nearby stars such as Iota Draconis. The Europeans are similarly hoping to build such a giant telescope called "Darwin." We all hope that Europe and the U.S. will join with Japan, China, and other countries to build this marvelous, spaceborn telescope that can analyze the atmospheres of other planets.

[Editor's Note: For more on how the atmosphere of a distant planet could reveal signs of life, see Detecting Life Beyond Earth.]