Earth's twin is out there -- somewhere

For the last 20 years, UC Berkeley astronomer Geoff Marcy has been the world's leading planet finder. Of the 260-odd planets that have been discovered in other solar systems, Marcy and his team have found 150. His most recent discovery, announced last week, is a fifth planet orbiting a star called 55 Cancri, about 41 light-years from Earth. Marcy, 53, sat down in his office to talk about the friendly and not-so-friendly competition to find the first Earth-like planet that could harbor life.

Describe your latest discovery.

This is one of the nearest stars to our sun. It has nearly the same mass as our sun, the same temperature as our sun and the same age. Frankly, what's delightful about it is that we now have five major planets orbiting it. The planets around 55 Cancri have a range of masses, from around 10 Earth masses at the smallest, to the largest, which is around four times as large as Jupiter. It's certainly the largest complement of planets ever found around another star.

Are any of these planets habitable?

This newest planet, No. 5, resides in the habitable zone, about 0.8 Earth-sun distances from its star. So this new planet we've found would be warmed up -- like a face to a campfire -- to lukewarm temperatures, making the water, if any, liquid. Having said that, we suspect that this new planet is made mostly of hydrogen and helium gas. Its mass is about 55 times the mass of Earth. So it probably isn't just a solid rock, like our Earth. Such a big planet with a rocky core and a fluffy gaseous envelope probably can't support life as we know it.

How many planets have you discovered?

My team has discovered 150. The Swiss team is a strong second. In fact, I just got an e-mail from the leader of the team congratulating me on the five planets around 55 Cancri.

So the competition is friendly?

It's a touchy friendship. We laugh about it. But in the true spirit of science we appreciate the competition because we know if we snooze, we will lose the next precious planets that are the next exciting batch to find.

What's the allure of an Earth-like planet?

To find the first Earth was a dream of Aristotle. Even in the religious realm, people have wondered, and still do, whether Earth was uniquely put here. Not to delve into touchy issues, but there's still a large -- how shall I put it -- spiritual question. Is the Earth the center of creation? And we're about to find out whether there are any other Earths out there. The Vatican will be interested. It's no joke. I've gotten two calls from them.