How astronomers learned our galaxy is jam-packed with planets

SCIENCE by Gregory Beatty

© Chris Mulligan

When Star Wars opened in 1977, the idea that planets like Alderaan and Tatooine could exist in far-off solar systems was pure science-fiction.

Now, hardly a week passes without someone announcing they’ve discovered a new planet. There’s even a fancy science-word for these faraway worlds — exoplanets.

October marked the 20th anniversary of the first confirmed exoplanet: 51 Pegasi b. Since then, close to 2,000 additional worlds have been found — the majority by the Kepler Space Telescope, which NASA launched in 2009.

Because of the vast distances involved, and the disparity between stars (huge, very bright) and planets (relatively tiny, not bright), astronomers have to be crafty to find exoplanets. One technique involves looking for small Doppler shifts in a star’s light that suggest its position relative to Earth is being impacted by an orbiting body.

Another is to look for fluctuations in light strength as an exoplanet “transits” across a star’s face.

“When a planet moves in front of a star it blocks a tiny amount of light,” says University of Saskatchewan astronomer Stan Shadick. “By taking a series of photos as this is happening, you can measure the drop in light.

“You can calculate the planet’s size and see if it’s large, like Jupiter, or smaller, like Earth,” adds Shadick. “There are also more sophisticated astronomical techniques where you can measure the mass ratio of the planet to the star. That allows you to calculate the density, and that tells you whether it’s rocky or gaseous.”

We’ve known for decades that stars and star systems come in a variety of sizes and configurations. Some stars are much larger and brighter than the Sun, and burn out in mere millions of years. Others are smaller and dimmer and have trillion year potential lifespans.

Similarly, while our Sun is solitary, many stars exist in binary and multi-star systems.

From what we can tell, the planets that orbit other stars are equally diverse — and some have confounded our understanding of planetary science. For example: because Jupiter and Saturn are far from the Sun, we used to think gas giants had to be far away from a star to form and stay intact. Nope! We’ve found so-called “Hot Jupiters” which orbit their stars closer and faster than even Mercury, our own solar system’s innermost planet.

Earth-like planets have also been found. And that, of course, raises the question of whether there are any real-life equivalents of Ewoks and Wookiees out there.

Jennifer Bentz is a Queens University PhD student in geological sciences and engineering. For her, the answer is clear.

“I really believe there’s life out there. It seems implausible that there isn’t,” she says.

“Life needs three things to survive,” says Bentz. “You need liquid water. That’s one thing they’re searching for on Mars. It used to have water on the surface, there’s lots of evidence of that. You need water because it facilitates chemical reactions that all organic matter uses to support life. It’s a universal solvent too, that dissolves pretty much anything except fats and oils.

“The second thing you need are elements critical for biological processes,” Bentz adds. “Carbon’s the important one, because it forms all of our organic molecules. It’s special because it can form four covalent bonds, so it can make a wide variety of compounds. And it’s small, so it fits into lots of compounds.”

The third thing you need for life, says Bentz, is an energy source. On Earth, that’s provided primarily by light and heat from the Sun.

When they’re assessing the prospects for life beyond Earth, astrobiologists speak about a “habitable”, or “Goldilocks” zone where it’s neither too hot nor too cold for liquid water to exist. That said, ongoing investigations into our planet’s extremophile lifeforms — such as microbes at the bottom of the ocean that feed on hydrogen sulfide from volcanic vents that support a thriving ecosystem — is forcing us to rethink our definition of “habitable”.

And that is causing us to take a second look at certain planets and moons in our solar system.

“I study Mars, and I really believe it used to have life — probably microbial,” says Bentz. “But it [likely] doesn’t any more. There’s no real liquid water that things can access. The surface has too much UV radiation, too. But we’re looking for life in the fossil record.”

The exact process by which life emerged on Earth is unclear. But evidence suggests that within a few hundred million years of our planet forming 4.5 billion years ago, life was present.

Not that evolution was easy, says Bentz.

“If life lived on Mars, it obviously didn’t last long because conditions weren’t right. Mars didn’t have enough gravity to hold the water [or] atmosphere that was needed to protect it. Even here, life probably had to evolve a few times. Early in our history there was a heavy bombardment period [by comets and meteorites] that probably disrupted evolution, and life had to form a few times before conditions stabilized.”

Astrobiologists haven’t ruled out the possibility of life existing deep underground on Mars. Jupiter’s moon Europa, and Saturn’s moons Titan and Enceladus are also prime candidates. They’re ice-covered, but tidal heating from their host planet’s gravity and volcanic vents could have given them sub-surface oceans.

And where you have liquid water, organic chemicals, and energy, as Bentz said, you have the possibility for life.

While we’re not going to find true intelligent life in our solar system, each discovery of an Earth-like exoplanet in a “habitable zone” makes the odds more favourable that we’ll one day encounter the real-life equivalent of Chewbacca or (heaven forbid) Jar Jar Binks.

Still, while life had a toehold on Earth 4.2 billion years ago, it wasn’t until 2.1 billion years ago that multi-cellular life emerged — and primitive animals like jellyfish didn’t evolve until 580 million years ago. At any step in that long journey, catastrophe could have struck.

Life isn’t easy, whether it’s on Earth, Kashyyyk or 51 Pegasi b. So for now, the Star Wars universe of Rodians, Ithorians and Mon Calamari only exists in our imaginations.

It’s probably just as well. We’ve had all the Gungans we need for one lifetime.

Greg Beatty’s editor helped him with some of this story’s nerdy details.