A potentially Earth-like planet circles a bright star 150 light-years away, casting a shadow tracked from space - and now from Earth, too. The planet, called K2-3d, was first seen crossing in front of its stars by NASA's Kepler space telescope during an ongoing K2 experiment mission.

NASA researchers brought the Okayama Astrophysical Observatory's 188-centimetre telescope to bear on the speck to fine-tune their understanding of the exoplanet's orbit down to a precision of 18 seconds, sources revealed.

By using the first ever Earth-based measurement, researchers predicted when the planet will cross its star in 2018 when the newly complete James Webb Space Telescope should be able to watch it carefully and analyze it for signs of life and habitability.

K2-3d is 1.5 times the size of Earth and orbits a star half the size of the sun every 45 days. The exoplanet circles closer to its star than Earth does around the sun, one-fifth the Earth-sun distance, but because this is a cooler star, the planet should rest at an Earth-like temperature that could host liquid water, National Astronomical Observatory of Japan researchers said in a statement.

Kepler discovered the planet based on the star's slight dimming as the planet passed in front, from the telescope's perspective - a process of discovery called the transit method. Because of Kepler's K2 mission, researchers were able to observe K2-d crossing the star just twice.

But the planet is closer to Earth and has a brighter host star than most of the other potentially habitable planets discovered by Kepler, researchers said in the statement. NASA's Spitzer Space Telescope observed two more transits, further increasing what researchers knew about the planet's orbit. 

Despite its closeness to Earth, K2-3d is small enough that the dip in brightness of its star is still extremely faint, only a 0.07 percent change.  Each transit presents a chance to learn more about K2-3d, and this new work allows researchers to forecast when those transits will occur.

Upcoming large-scale telescopes like the James Webb Telescope will be able to analyze the starlight shining through the planet's atmosphere during a transit. They'll thus detect molecules like Oxygen that can indicate life on that planet.

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