The search for new worlds goes into interstellar overdrive
Science | by Gregory Beatty
From Nicolaus Copernicus’ 16th century discovery that Earth, contrary to Christian doctrine, wasn’t the centre of Creation but instead revolved around the Sun, to William Herschel’s discovery of the first non-naked eye planet, Uranus, in 1781, astronomers have made a habit of blowing people’s minds over the centuries.
They haven’t run out of tricks. Just 25 years ago, they detected the first exoplanet.
Exoplanets are planets beyond our solar system. They’re usually found around a star (or stars), although there are rogue exoplanets out there.
Exoplanets raise the possibility of life existing elsewhere in the Milky Way Galaxy, so their discovery was another mind-blowing milestone.
I learned this, and more, when I interviewed University of Saskatchewan astronomers Stan Shadick and Daryl Janzen about a March 8 talk they’re giving in Saskatoon: Exoplanets and the Search for Habitable Worlds.
Given what we know of our own solar system’s formation it was no surprise exoplanets exist, says Janzen.
“As long as you have a gas cloud that’s full of heavier elements it’s quite common,” Janzen says. “Sometimes a star might form with nothing around it, but more likely the gas flattens into a disk where matter can accumulate into planets.”
They might be common but because of the light-year distances involved and the glare cast by the host star(s), it’s hard to find exoplanets. Astronomers use two indirect methods to detect them. One involves searching for wobbles in a star’s light spectrum, which indicates it’s being perturbed by revolving planets. The other looks for variations in light intensity caused by a planet passing in front of a star (an event called a transit) and temporarily blocking some of its light.
“It’s less probable for a planet to be discovered [through transits] because it has to pass in front of the star from our line of sight,” says Janzen. “With the first method, the planet doesn’t have to transit for there to be a perceivable wobble.”
While transit discoveries are rarer, Shadick says they give us more information.
“It’s only by studying transits that we can measure the planet’s radius, which can give us the mass and density and determine whether it’s a gas giant, like Jupiter or Saturn, or a rocky planet like Earth or Mars,” he says.
So far, we’ve detected over 3,700 exoplanets in 2,700-plus star systems. The search was helped by the Kepler Space Telescope, which NASA launched in 2009. Based on the planets it has found in the limited range of sky it studies, astronomers estimate there are 40 billion rocky, Earth-size worlds in habitable-zone orbits around Sun-like and red dwarf stars in the galaxy.
Now, about that habitable zone…
The Goldilocks Principle
When we think about the prospect of life existing elsewhere in the galaxy, we’re held back by a lack of information. After all, the only example we have to go on is our own.
Still, certain parallels seem inevitable, Shadick says.
“One key is liquid water,” says Shadick. “It seems to be a primary ingredient for life. Every organism, to some extent, needs water to transport nutrients in its body.
“That’s where the Goldilocks principle comes in,” he says.
“Some planets are too hot because they’re too close to their star for liquid water to easily exist. And other planets are too far from the star, so water is frozen.”
Right. Some are too hot, and others are too cold. But it takes more than liquid water to make a planet ‘just right’ for life.
Water is made of hydrogen and oxygen, which just happen to be two of the five most common elements on the periodic table along with helium, carbon and nitrogen. Life on Earth is carbon-based, but again, that’s not surprising. Not only is carbon abundant, it has special chemical properties that allow it to form four covalent bonds, which means it’s well-suited to form complex compounds, such as amino acids — the building blocks of life.
Two guesses, too, which elements are most common in Earth’s atmosphere: nitrogen (78 per cent) and oxygen (21 per cent).
Even in our own solar system, though, the idea of using Earth as a model of a habitable planet comes with a qualifier, says Janzen.
“As we continue to explore, we’re discovering that our idea of Earth being a special planet in the Goldilocks zone may not be the only possibility for life,” says Janzen. “With the ice moons Europa and Enceladus at Jupiter and Saturn, tidal heating may have permitted the evolution of life [in subsurface oceans].
“It wouldn’t be life like we know it — it would probably be microbial.”
Earth’s Goldilocks conditions aren’t the only factor that helped advanced life evolve, says Janzen.
“Earth is part of a system that has four large outer planets [Jupiter, Saturn, Uranus and Neptune] that have knocked a lot of comets and asteroids out of the way so that we haven’t been continuously bombarded throughout our existence,” he says.
“The Moon, too, has played a huge role in stabilizing Earth’s rotation so that we’ve had a steady day and seasons for billions of years.”
The Next Telescope
While Kepler has performed valiantly, it’s had several technical failures. The good news: in April, NASA and SpaceX are scheduled to launch the Transiting Exoplanet Survey Satellite —TESS for short.
TESS will survey near-Earth stars for transiting exoplanets, and identify prime targets for future investigation with large ground-based and orbital telescopes — including the James Webb Telescope, scheduled to launch in 2019.
The Holy Grail will be the discovery of life on an exoplanet. Because of the vast distances involved, that’s most likely to come through the detection of ozone, says Janzen.
“From our understanding of how Earth evolved, as bacterial life started to emerge oxygen was produced and that caused the atmosphere’s composition to change dramatically,” he says. “It was after that that more complex lifeforms evolved.”
While satellites like Kepler and TESS have grabbed most of the headlines, Shadick says important exoplanet research is being done at the local level.
“We like to focus on large telescopes and a lot of key discoveries are made with them,” Shadick says. “But they’re expensive to operate, and you can’t use them for long-term projects. Because exoplanet transits can be studied with smaller telescopes — like those at the university — we can study them night after night. That allows us to accumulate large data sets that might be necessary for the eventual discovery of an exomoon.”
Right now astronomers are discovering exoplanets like Easter eggs in a playground hunt. They’ve found so many that it’s fuelling science fiction fantasies of us exploring and colonizing other worlds.
The reality, says Shadick, is more sobering. And it carries an important message.
“Yes, we’re finding lots of exoplanets,” says Shadick. “But most are not at the right distance from the star, or they’re the wrong mass to be a planet with a rocky surface that’s large enough to hold onto an atmosphere.
“Earth-like planets are really rare, and I’m not sure it’s going to be easy to move our civilization anywhere else,” concludes Shadick. “It’s much more important that we look after this planet we’re inhabiting now and not assume we can go elsewhere if we mess things up.”