Searching for sister Earth

Earth and Kepler-452b

By Travis Metcalfe

In last year’s blockbuster movie Interstellar, Matthew McConaughey leads a team of astronauts on a quest to find a new planet like Earth, after humanity collectively destroys the climate on the only home we’ve got.

Setting aside the feasibility of interstellar space travel, the good news is that astronomers have made great strides over the past two decades in their search for planets like Earth around other stars. Some of the most important breakthroughs have actually occurred right here in Boulder, and scientists at University of Colorado Boulder are currently developing technologies that will help measure the atmospheres of Earth-like planets in the future.

For most of history, we only knew about five other worlds in our solar system: Mercury, Venus, Mars, Jupiter and Saturn. The Greeks called them “planets” (a word meaning “wanderers”) because these bright lights in the sky were constantly moving against the backdrop of fixed stars. The more distant planets had to wait for the invention of the telescope. Uranus was discovered by a British astronomer in 1781, while the existence of Neptune was predicted by a French mathematician and confirmed in 1846. The first planet around another star like the Sun wasn’t discovered until 1995. It was comparable in size to Jupiter but 100 times closer to its sun, exerting a tiny gravitational tug that astronomers could measure.

By 1999, dozens of planets had been discovered around other stars, and for the first time astronomers found one that passed directly in front of its parent star, causing a brief eclipse of the starlight. Harvard graduate student David Charbonneau was working on his doctoral thesis in Boulder with Timothy Brown, then a senior scientist at the National Center for Atmospheric Research (NCAR). Brown had built a specialized instrument for the project — on a tight budget he had even ground the telescope optics himself in his garage in Louisville. Together they used the instrument to survey some stars that were suspected of having large planets, hoping that one of them would fortuitously be aligned such that the planet would periodically pass in front of its star.

They set up the telescope in the back parking lot of the NCAR laboratories, and in late 1999 they saw a Jupiter-sized planet pass in front of its star on two separate occasions.

This discovery paved the way for NASA’s Kepler space telescope, which was approved just two years later. The mission was designed to find small planets like the Earth using the technique pioneered by Brown and Charbonneau.

Kepler was built by Ball Aerospace in Boulder. It is the size of a small car, with a mirror less than half the diameter of Hubble’s. It used a 95 megapixel digital camera to snap new images every 30 minutes and measure the brightness of 150,000 stars, hoping that a few percent would show tiny eclipses from planets.

After launching from Cape Canaveral in 2009, Kepler spent the next four years staring at one small patch of the summer Milky Way. A planet like Earth would only block out one-tenth of a percent of the starlight for several hours as it passed in front of its star once a year. Kepler provided an unblinking eye to discover these small planets, and it found them in droves. Among the thousands of planets it discovered, about a dozen are nearly as small as Earth and orbit at a distance from their star where liquid water could exist. These planets are just from the 150,000 stars that Kepler surveyed. Our galaxy has a few hundred billion stars, so there must be millions of planets similar to the Earth!

Just because a planet is small and orbits at the right distance from its star doesn’t mean it would be a nice place to live. To get a better idea of how hospitable these planets might be, scientists at CU’s Center for Astrophysics and Space Astronomy (CASA) are working on a way to get direct images of the newly discovered worlds.

The basic concept has already been demonstrated. Block out the light of the star and look for light from the orbiting planets. This is fine for large planets like Jupiter that orbit far from their star. But to get images of Earth-like planets that are 10 billion times fainter than their sun, you need a “star-shade” — a large disk shaped like a flower that can position itself between the star and a telescope. Some engineering challenges remain, but CASA scientists Anthony Harness and Webster Cash hope to see the first images of Earth-like planets within a decade. Pass that light through a prism, and you can probe the composition of the atmosphere, maybe even see the signature of green plants.

As in the movie Interstellar, it’s been a long journey to find other planets like Earth. Without ever leaving our planet, astronomers and engineers around Boulder have pushed forward the discovery that small planets are the rule rather than the exception, and that potentially habitable worlds are peppered throughout the galaxy. The technology to measure their atmospheres is just around the corner. But even after we’ve found sister Earth, the prospects for mass migration are dim. Better not wreck our own planet just yet.