Mr. Kant’s Planets Come to Light

This spring, astronomers fulfilled a scientific quest -- and made theory concrete -- by capturing the first direct images of planets beyond our solar system. It’s time to rewrite the science books -- instead of nine planets, there are many more, and we can see some of the “new” ones with our own eyes.

First predicted by the German philosopher Immanuel Kant 250 years ago, alien, or “extrasolar” planets, have been tantalizing but elusive targets, objects of scientific speculation rather than fact. It wasn’t until about a decade ago that we had any evidence of their existence. Even then, it was indirect evidence -- astronomers observed not the planets themselves but the gravitational tug on the motion of their parent stars. Since then, about 150 planets have been identified in this way.

Then, in March, NASA astronomers took us much closer to two of them when they succeeded in capturing the images -- more precisely, the reflected infrared light -- of Jupiter-like planets near sun-like stars. And once they captured the light they could analyze it and say what molecules were present in the atmospheres of these planets. For example, the NASA astronomers tell us that one of the newly photographed planets probably has water in its atmosphere.

It’s a triumphant confirmation of Kant’s centuries-old hypothesis. In 1755, only six planets were known: Earth, Mercury, Venus, Mars, Jupiter and Saturn. Kant initially predicted that more planets lay beyond Saturn. Then, on realizing that our solar system was tucked away in an undistinguished corner of the Milky Way galaxy, he also advanced the idea that other planets must exist outside our solar system.

The first of Kant’s predictions came true during his lifetime. In one of those happy accidents that have been part of the progress of science, Uranus was discovered by chance in 1781. That caused dozens of amateur astronomers to train their telescopes on the night sky in the hope of being the next planet finder. The quest was immortalized in poetry by John Keats: “Then felt I like some watcher of the skies/When a new planet swims into his ken.”

But it took 65 years before the next planet, Neptune, was discovered, in 1846. And then, 84 years later, after a laborious search of the heavens, the faint Pluto was found in 1930. The inventory of the sun’s planets was complete.

The reason it took decades longer to fulfill the rest of Kant’s prediction is that looking for planets beyond the solar system is a complex problem. The separation between stars is measured not in miles but light-years, a light-year equaling almost 6 trillion miles. As a consequence, even planets orbiting nearby stars a few light-years away are incredibly faint to our telescopes. Not only is identifying the faint planet next to the bright star a challenge, but in its voyage across interstellar space, reflected planetary light is so diminished that getting it on camera becomes an almost impossible task.

What do the new planets look like? Faint, hazy smudges -- but they’re definitely there, Jupiter’s cousins, light-years away. NASA’s announcement has been followed by reports from other teams of astronomers, which have images that are more controversial but may also turn out to show new extrasolar planets. Our technology is finally good enough to usher in what may be a golden age of discovery.

Of course, the question that follows is will we be able to see planets like the Earth out there? After all, the sun is an ordinary star, and there are millions of similar stars in our galaxy. If there are Jupiter-like planets around sun-like stars, is it not conceivable that they also have Earth-like planets orbiting them?

Searching for direct evidence of Earth-like planets -- Earth is much smaller than Jupiter -- will take a new generation of powerful telescopes. But surely we will succeed. And then, of course, we will have to deal with the biggest question of all: Is there other life out there?

Maybe in another 250 years, we will know.