If our solar system is not unusual, then there are so many planets in the universe that, for example, they outnumber the sum of all sounds and words ever uttered by every human who has ever lived. To declare that Earth must be the only planet with life in the universe would be inexcusably bigheaded of us.

This influential, yet controversial idea requires that the mixture of species on Earth at any moment acts as a collective organism that continuously (yet unwittingly) tunes Earth's atmospheric composition and climate to promote the presence of life... But I'd bet there are some dead Martians and Venusians who advanced the same theory about their own planets a billion years ago.

When I reach to the edge of the universe, I do so knowing that along some paths of cosmic discovery, there are times when, at least for now, one must be content to love the questions themselves.

I don't want to go back into space for military reasons, but the economic driver still remains. And so it's a matter of people understanding how that economic driver is revealed with healthy investments on the space frontier.

The chances that your tombstone will read 'Killed by Asteroid' are about the same as they'd be for 'Killed in Airplane Crash.'

If we have the power to turn another planet into Earth, then we have the power to turn Earth back into Earth.

No matter what eyewitness testimony is in the court of law, it is the lowest form of evidence in the court of science.