Ben Bova: Are there other Earths? The search goes on

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In all the wide and starry universe, are there other worlds like our own? Can we ever hope to find new Earths?

Or is our world somehow unique?

We know that none of the planets in our own solar system is very much like Earth. Mars comes closest, but it’s a frigid desert from pole to pole, where temperatures rarely get above freezing and overnight lows are usually 100 degrees below zero or worse.

Venus has been called our “sister planet” because it’s about the same size as Earth. But Venus is a blazing-hot, dry world where ground temperatures are high enough to melt aluminum and the sky is perpetually covered with clouds of sulfuric acid. Some sister!

To find a new Earth, we must look much deeper into space, and seek planets orbiting other stars. Which brings up a question: Do other stars harbor planets, or is our solar system a rare oddity in the universe, perhaps even unique?

Finding planets around other stars is not easy. The stars are enormously far away, their distances measured in light-years — the distance light travels in a year. Moving at 186,000 miles per second, light from the sun reaches Earth in eight minutes. It takes light some 5 1⁄2 hours to go from the sun to Pluto, on the outskirts of our solar system.

To reach the star nearest our solar system, Alpha Centauri, light must travel for 4.3 years. If the distance between the Earth and sun (93 million miles) were shrunk to one inch, Alpha Centauri would be 4.3 miles away!

Moreover, while stars are bright, shining from the energy they generate in their cores, planets are dim and shine only in the starlight they reflect.

Looking for planets around other stars has been compared to finding a firefly fluttering around a searchlight. Not easy.

Although astronomers cannot see planets around other stars, they have found ways to detect them. Planets exert a gravitational pull on their stars. The tug is quite small, like the impact of a flea on an elephant’s rump, but it can be measured — in a precious few cases.

In 1995 astronomers discovered a planet orbiting a sun-like star in the constellation Pegasus. In short order, dozens more discoveries came out, all of them based on measuring the teeny perturbations of the star’s motion caused by its planetary companion’s gravitational pull. As of today, more than 340 extrasolar planets have been found.

Our solar system is not alone. But while other stars do indeed harbor planets, almost all of the exoplanets are very much unlike Earth.

Most of them are “hot Jupiters,” enormous gas giant worlds hundreds of times more massive than Earth, yet orbiting so close to their stars that their “years” take only a few days to complete and their surface temperatures are far above the boiling point of water.

It looks as if hot Jupiters are abundant and Earth-sized planets vanishingly rare. But looks can be deceiving.

The technique of detecting extrasolar planets from the gravitational tugs they exert on their parent stars makes it easiest to find large, massive planets spinning very close to their stars. Like the blind men and the elephant, we are seeing only a small part of the total picture. Too soon to draw conclusions.

Indeed, a few smaller exoplanets have been detected. The Swiss team that discovered the first extrasolar planet back in 1995 recently found a planet that’s two to four times the size of Earth. But it’s so close to its parent star that it’s far too hot for liquid water to exist on its surface. Liquid water is a requirement for our type of life.

There’s another way to detect exoplanets. When a planet passes in front of its star, the star’s light dims a little. Very little. But sometimes …

In 2006 France launched COROT, a spacecraft intended to find extrasolar planets by measuring fluctuations in stars’ brightnesses. Earlier this year NASA launched the Kepler spacecraft, which carries a much more sensitive system for doing the same thing.

A few months ago COROT detected a planet that’s only about twice the size of Earth, although it’s so close to its star that it can’t hold liquid water.

Kepler will be looking at thousands of stars, and its sensors should be able to pick up Earth-sized planets easily. I’m betting that before this year is out we’ll get reports of many Earth-sized exoplanets.

What then? Realize that neither COROT nor Kepler can give us visual images of the planets they detect. For that we will have to wait for NASA’s James Webb Telescope, designed to replace the aging Hubble Space Telescope. NASA also wants to build the Terrestrial Planet Finder (TPF), a more complex set of telescopes to be placed in space specifically to give us pictures of Earth-sized exoplanets.

And not just pictures. TPF will be able to detect oxygen and water vapor in those distant worlds’ atmospheres, if those ingredients for life exist there.

We know that our solar system is not the only one in the universe. Planets seem to be almost as abundant as stars. Soon we will know that Earth-sized planets are out there, as well. And then we will search for evidence of life on those worlds.

But all these possibilities depend on the budgets that NASA receives from Washington. The Webb Telescope and TPF could be canceled at any time. Both projects will cost more than a billion dollars.

Is it worth that much money to learn if there are other Earth-type worlds among the stars?

Are we willing to do what it takes to find out that we’re not alone in the universe?

Naples resident Ben Bova is the author of 120 books, including “Faint Echoes, Distant Stars,” a nonfiction study of the search for life on other worlds. Bova’s Web site address is www.benbova.com

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