When musician and amateur astronomer William Herschel first spotted the planet Uranus through his homemade telescope back in 1781, he believed he had discovered a comet. But right from the start, he knew it was no ordinary comet: it had no tail, and everyone knew comets had tails.

Herschel was both wrong and right. He was wrong to believe that no planets existed beyond the original five that the ancient Greeks had known. But he was right to believe that only comets have tails (at least when they're within close distance of the sun) and that planets never do.

That's the way it works in our solar system anyway. But since astronomers began discovering worlds orbiting other stars in the 1990s, it's become clear that our family of planets isn't the only possible kind. The first planets ever found outside of our solar system, called exoplanets, shattered that belief. The new worlds were huge, like Jupiter, but hugged their stars so tightly that they completed a full orbit in just a few days.

And now, one of these so-called "hot Jupiters" has also demolished the idea that planets don't have tails.

Growing a Tail

In 1999, astronomers discovered a world, about 150 light years from Earth, called HD 209458. Careful measurements by the Hubble telescope of the exoplanet show that its atmosphere is continually giving off hot gas. The gas is then caught by the star's powerful particle wind and shaped into a tail.

HD 209458 is so close to its star that its blanket of gas heats up to a searing 2,000°F or so. The scorching heat drives winds of 7,000 miles per hour and forces the outer layers of gas to spew into space like steam boiling from a teakettle. That ongoing blast keeps the planet's tail burning. Eventually, this process will burn the planet away to nothing, but that would take approximately a trillion years. By then, the star it orbits will be
long gone.

Something in Common

Astronomers have been watching this planet carefully ever since it was first spotted. The planet crosses directly in front of its star once every orbit, which happens every three days or so. Starlight passes through the planet's atmosphere on its way to Earth. By studying how the light changes, scientists have learned that the planet's atmosphere contains gases similar to those in our own atmosphere.

A tail coming from a planet is certainly a curiosity. Using a new instrument that was recently installed on the Hubble telescope, scientists could gather all of the information they need without ever seeing the planet directly. With devices like this and other advanced technologies, astronomers hope to probe smaller, more Earth-like planets to see if their atmospheres might be similar to ours and if they bear the chemical makeup needed for life to exist.

In short, this hot, comet-like planet is just the beginning.