“Is it true that the starlight we see is millions of years old?” one of my students asked me this semester. “So that when we look at the sky we’re actually looking back into the past?”
It was a cold, clear night in Boston, and I was teaching a night lab session on the roof the department building, observing with a few 8-inch telescopes.
“Exactly,” I said, with a big grin on my face. He’d hit upon one of the most poetic truths of astronomy. I spoke loudly enough for the rest of the students on the roof to hear, switching into lecture mode. “Light can’t travel infinitely fast, so it takes some time to reach us. We’re not seeing the stars as they are now, we’re seeing them as they once were—years, centuries, maybe tens of thousands of years ago. The light from the galaxies you can see through the telescopes is millions of years old, or older.”
His response genuinely surprised me. “That’s so depressing!”
“Really?” I asked. “Why?”
“Because there could be other worlds out there right now—other life, other civilizations!—and we’ll die never knowing they were there. It’ll take us millions of years to see it! We chose the wrong time to be alive.”
I’d never thought about it that way. Jeez, I thought, that is depressing. I stood speechless for a few seconds, staring off into space.
“Well,” I finally replied, “that’s one way to look at it. But consider what we’re discovering right now. Did Professor West go over the Kepler results in lecture?”
Scientists working on NASA’s Kepler space telescope had just announced their newest findings that week in their search for planets around other stars.
“Yeah,” he nodded. “They found like like 1200 new planets?”
“Yup,” I said, “and 68 of them were earth-sized and rocky, and 54 of them were in the habitable zone”—that is, not too hot and not too cold: just right to support liquid water on its surface. “So put that in perspective. That’s a huge shift in how we see ourselves as a species. Think about it—for 100 million years humans lived not knowing if the earth was special, if it was the only planet like it in the universe. And now, just this week we suddenly know of 54 planets that can support water, that could potentially support life. Fifty-four potential worlds. Before this week, we only knew of one. And for the 100 million years of the human race before that, we didn’t know of any. So in a sense, we’ve chosen an extremely exciting time to be alive. And exactly the right time to take this class.”
He nodded, but I could tell he wasn’t convinced. He turned his head skyward and gazed forlornly at…