One of the defining moments of the scientific revolution occurred 400 years ago. In the spring of 1609, Johannes Kepler published Astronomia nova, which provided strong evidence for Copernicus’ heliocentric model of the universe. In the fall of the same year, Galileo used a telescope to observe the night sky for the first time.
In recognition of these seminal events, the United Nations declared 2009 the International Year of Astronomy, and McGill University’s Faculty of Science has organized a triumvirate of commemorative public lectures entitled “Black Holes, New Worlds, and the Universe.” Sara Seager, a professor at the Massachusetts Institute of Technology (MIT), presented the first of these lectures, “Origins and Aliens: The Search for Other Earths,” on September 21 to a near-capacity crowd in Leacock 132.
Seager’s research focuses on trying to find and analyze exoplanets – planets outside our solar system that orbit a star, and which are most likely to harbour alien life. Seager, an expert in analyzing the way that light diffracts in an alien atmosphere to reveal the gas composition, calls the resulting pattern formations the “fingerprints of a planet.” These fingerprints are useful for detecting tell-tale signs of life like oxygen, carbon dioxide, and water vapour.
“Oxygen is such a reactive gas, and if you find that an atmosphere has 20 per cent oxygen [as Earth’s does], you have to ask, ‘Why should it be there?’” Seager said.
The main difficulty in searching for hospitable exoplanets, according to Seager, is that the glare from stars obscures planets with Earth-like orbits. To illustrate this problem, consider that Earth is 10 billion times fainter than the Sun, meaning that our current ability to resolve planets vanishes at about 20 times the Earth-Sun distance. The ability to directly see an exoplanet with an Earth-like orbit would demand a massive, near-perfect space telescope, which would use non-circular mirrors to reduce the amount of glare observed.
Alternatively, a screen could be set up to diffract sun rays to a smaller, non-perfect telescope. But this solution also has its shortcomings, as it would require extreme spatial coordination between the 15 metre-wide screen and the telescope, which would be tens of thousands of kilometres apart.
Fortunately, there are other ways to find an Earth-like exoplanet. The “wobble method” and “transit method” are two indirect detection techniques that measure either the gravitational attraction between a planet and a star, or the change in a star’s radiation pattern caused by a planet’s orbit. From these observations, one can infer the mass, size, and temperature of a planet, but not the presence of biological signs.
“These [indirect] methods can’t tell us the composition of the atmosphere,” said Seager.
According to Seager, exoplanet travel will not be happening anytime soon. The possibility of visiting an exoplanet is a venture currently hindered by considerable economic and logistical barriers. The closest star to Earth is Alpha Centauri, approximately four light-years away, and though we might consider the claim that humans could travel at one-tenth the speed of light, such a journey would still take 80 years, round-trip.
We can still learn a lot about planets by studying them from afar. Current research can identify planets’ densities, atmospheres, and the different phenomena that they experience – for example, how heat is transferred across tidally-locked objects like Earth’s moon.
As for finding an Earth-counterpart out there, Seager is optimistic that she will witness the historic event. “People often ask me whether we’ll find another Earth in my lifetime,” she said. “And I say ‘Yes. But I expect to live a very long time.’”
Realistically, Seager estimates, such a planet will be discovered within 20 to 25 years. Whether little green men will be waiting there is another question.
The next lecture in the series is entitled “The Origin of the Universe and the Arrow of Time” and will be presented by Professor Sean Carroll on October 19, 6 p.m. in Leacock 132.