We’ve come a long way. First it was only giant planets seared by hot proximity to their host stars. Not anymore. Astronomers announced the discovery today of the first Earth-sized planet in the “habitable zone” of its host star. Habitable, when it comes to extra-solar planets, means the right distance for liquid water to pool on the planet’s surface. Water is intimately tied to life on Earth and may also be on other worlds.
Other planets have been discovered in their stars’ habitable zones, but they’ve all been at least 40% larger than Earth. This one, named Kepler-186f, nearly matches Earth in size and orbits Kepler-186, a star with four previously known planets 500 light years away in the constellation Cygnus (Northern Cross).
“Being in the habitable zone does not mean we know this planet is habitable,” cautions Thomas Barclay, a research scientist at the Bay Area Environmental Research Institute at Ames, and co-author of the paper. “The temperature on the planet is strongly dependent on what kind of atmosphere the planet has. Kepler-186f can be thought of as an Earth-cousin rather than an Earth-twin. It has many properties that resemble Earth.”
Kepler-186f is 10% larger than Earth and orbits an M class red dwarf star every 130 days, soaking in just one-third the amount of heat Earth receives from the sun. That places it near the edge of the habitable zone. Its mass and composition are still unknown, but previous studies imply planets this size are made of rock like Earth.
Transiting Exoplanet Survey Satellite (TESS) – the next generation in exoplanet discovery
The Kepler space telescope infers the existence of a planet by the amount of starlight blocked when it passes in front of its host star. From these data, a planet’s size, orbital period and the amount of energy received from the host star can be determined. Before an equipment failure, Kepler observed nearly 150,000 stars simultaneously looking for dips in the stars’ light made by orbiting planets.
To date, about a thousand planets have been confirmed with the Kepler data with another nearly 3,000 unconfirmed candidates.
“Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind’s quest to find truly Earth-like worlds,” said Paul Hertz, NASA’s Astrophysics Division director.
Like we often do in our personal lives, we’re looking for someone like us. Earth’s twin is out there. We’re getting closer.