Is alien exists?

For as long as people have walked the Earth, they’ve wondered if there are other
places out there like this one—planets where other beings gaze in awe at the starry
sky.

Now for the first time in human history, we are on the verge of knowing the
answer. Soon, we may find other living worlds. Finding another planet like Earth is tremendously exciting for astronomers, and I hope for you too.

If our star, the Sun, has planets, shouldn’t other stars have planets, too? No one
knew the answer to this question until about the mid 1990s, when the first planets
were found around nearby stars. We call a planet orbiting a star other than the Sun
an “exoplanet.” Today, we’ve discovered over 400 of them, but none resemble the
Earth.

A trickier question is, “where in our galaxy can we find planets like Earth that we
can study for signs of life?”

 Surprisingly, the answer is, “also within the white circle.”
Beyond this, any stars and planets are too far away for us to study closely. Finding
other Earths is one of the most challenging tasks ever to face astronomers. But we
are working hard every day to make it happen.
The most fascinating thing about the hundreds of known exoplanets is their
huge variety. Some stars have a giant planet like Jupiter where the Earth would be.
Other stars have planets like Jupiter 10 times closer to them than Mercury is to our
Sun. Some stars have planets we call “super-Earths,” rocky worlds bigger than Earth
but smaller than Neptune. The list of bizarre planets goes on, and so far we’ve only
scratched the surface. If you can imagine a kind of planet—as long it falls under the laws of physics and chemistry—it’s probably out there, somewhere.

What could aliens see, Earth from a far?

If there is an alien civilization on a planet orbiting one of the 100 or so nearest Sun-
like stars, what could they learn about Earth?

This is a real picture of Earth taken by
the EPOXI spacecraft from more than 30 million miles away. This seems far, but it is
still nearly a million times closer than the nearest star. EPOXI used to be called Deep
Impact (when it dropped a “wrecking ball” into a comet on July 4, 2005). But it was
renamed when I and several other astronomers found a way to use the idle, drifting
spacecraft to study stars with planets and also to observe Earth as if it were an exo-
planet. From EPOXI’s images of Earth, we’ve learned how to estimate whether or
not an exoplanet has things like continents or oceans.
What would it take for an alien civilization in another solar system to take a
picture of Earth similar in quality to EPOXI’s? More than anything else, the aliens
would have to have a lot more money to spend on space telescopes than we Earth-
lings do; taking a picture like this across interstellar distances wouldn’t be cheap. It
would require about fifty telescopes in space all working together, each about half a
football field wide.

When will we find another Earth?

Let’s start with what it would take to find an “Earth twin,” a planet the same size and mass as Earth, with water oceans and an oxygen-rich atmosphere.

Rocky planets that could have surface liquid water are very small and dim com-pared to their large, bright, parent stars. Finding an Earth twin around a Sun-like star is like trying to see a firefly fluttering less than a foot from a huge searchlight—when the searchlight is 2600 miles away. This is the distance from New York to Los Ange-les, or the distance from London to Moscow. With a powerful telescope you might be able to see the firefly’s faint glimmer, but that glimmer becomes imperceptible in the
searchlight’s overpowering glare.
In numbers, Earth is 10 billion times fainter than the Sun at visible wavelengths. To understand this number, think about what you can buy for one dollar. Now think about what you can buy for 10 billion dollars. The problem in observing Earths is not so much the faintness of Earth—it is the glare of the adjacent, 10-billion-times
brighter star.

There are easier ways than direct imaging to find planets similar to Earth—although
finding Earth-like planets will never truly be “easy.”

The good news is that these alternate techniques will discover planets similar to Earth soon, in the next few years. The bad news is that the techniques won’t tell us if the planet is truly
Earth-like—differences between welcoming, life-bearing worlds like Earth and hostile, red-hot worlds like Venus would be indistinguishable. We will ultimately need to get a spectrum, a fingerprint of the planet’s atmosphere, to estimate whether the planet is habitable or inhospitable to life.

Another planet finding technique that might find an Earth-mass planet in the next
few years is the so-called “radial velocity technique.” The radial velocity technique
looks for stars that “wobble” along our line of sight. Such wobbles can be caused by
the gravity of orbiting planets, causing the star to move back and forth. Astronomers
can measure very tiny star wobbles—the same speed as you can walk or run—one
yard per second or less. At present, finding an Earth-mass planet in an Earth-like orbit is just beyond the technological limit of the radial velocity technique. But
astronomers are quietly collecting observations on a handful of very bright stars in
the sky. A momentous discovery could come any day.