Caleb Scharf is the director of astrobiology at Columbia University in New York City. Since receiving his PhD in astronomy from the University of Cambridge in 1994, he has become an internationally-known scientist, lecturer, and writer—the author of more than 100 scientific papers and the winner of the American Astronomical Society’s 2011 Chambliss Award for excellence in writing. His research includes the study of exoplanets and the search for life on other worlds; his latest book is “The Zoomable Universe.”
I spoke to Dr. Scharf about the ongoing pursuit of Earth-like planets in the galaxy, and the possibility of life elsewhere, particularly in the solar system. Here are excerpts from our conversation.
Bill Retherford: Is there an accepted definition for an Earth-like planet?
Caleb Scharf: The definitions we use for categorizing planets are kind of loose. The term Earth-like is a little tricky. When astronomers say ‘Earth-like,’ it can be a little deceptive. Most people think it means, ‘Hey, it’s a place where I can go to the beach.’
But in exoplanetary astronomy, we’re not necessarily talking about an Earth twin. We’re talking about an analogue world—a sister world that sha res some of the characteristics of Earth: a smallish rocky planet, about the same size, with a little bit of atmosphere, a little bit of water, and temperatures somewhere between the freezing and boiling point of water. Almost certainly it’s not going to be an exact twin.
BR: With that in mind, how many planets in the galaxy might be somewhat similar to Earth?
CS: It’s a question that astronomers are pulling their hair out trying to answer. Our estimates are pretty crude. It’s really hard to say. But out of the hundreds of billions of planets in the galaxy, maybe somewhere between 10 to 20 or 30 percent have the potential to be like the Earth. They almost certainly number in the tens of billions.
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