Earth's newfound 'cousin' planet could support life.
By Amanda Jermyn

Things are looking up for planet hunters with the recent discovery of Kepler 186f, the first Earth-sized planet found in the habitable zone of an alien star. The planet is named after NASA’s Kepler mission, which, in the five years since its launch, has confirmed the existence of 962 exoplanets, with over 2,900 candidates remaining to be studied. The Kepler orbiting telescope was designed to detect slight drops in brightness that indicate a planet crossing in front of its star. The discovery was made before Kepler suffered a mechanical failure last year, ending, at least for now, its planet-hunting days.

The newfound planet orbits the red dwarf star Kepler 186, about 500 light-years from Earth in the constellation Cygnus. It is about 10% larger than Earth, and orbits its star within the “Goldilocks zone,” the range of distance from a star where the temperature is just right for liquid water, and possibly life, to exist. “It has the right size and is at the right distance to have properties similar to our home planet,” said Elisa Quintana of NASA’s Ames Research Center, and lead author of the paper on the discovery published in the journal Science.

Kepler 186f is not alone in its solar system. Four other planets have been detected, all less than 1.5 times the size of Earth. However they orbit far too close to their star to support life.

While about 20 other exoplanets have been found in the habitable zones of their stars, they have all been much larger than Earth, and therefore more likely to be comprised mainly of gas, rather than rocky, like Earth. Researchers think Kepler 186f is likely made of similar materials to Earth, such a rock, iron, ice and liquid water, although perhaps in different proportions.

The newly discovered planet is being referred to as a cousin to Earth, rather than its twin, as conditions may be somewhat similar, but not the same. While Earth orbits the sun in about 365 days, Kepler 186f orbits its star in 130 days at a distance of about 32.5 million miles, similar to the orbit of Mercury around the sun. Mercury, of course, is far too hot to support life. However, our kindred planet’s star is a red dwarf, with about half the mass of the sun, and therefore much cooler than the sun. So it may in fact be cooler than Earth, with an average temperature just above freezing, but still within the habitable zone. With its slightly larger mass, however, it may have a denser, insulating atmosphere to compensate, making it similar in temperature to Earth. According to NASA researchers, on the planet’s surface at noon, its star’s brightness would be much like our sun’s brightness an hour before sunset, with an orange-red glow. This is enough light, however, to make photosynthesis in plants possible.

Knowing that a planet is in the habitable zone is, of course, a long way from knowing whether it is in fact inhabited. Our cousin planet most likely has liquid water, and is almost certainly rocky. It most likely has an atmosphere, probably with large amounts of carbon dioxide, but we can’t be sure of its composition. The chemical elements carbon, oxygen, nitrogen, hydrogen, sulfur and phosphorus have long been considered essential for life, but life on alien worlds may have developed differently from that on Earth, with different requirements. While we don’t know the age of this exoplanet’s star, we know that red dwarfs are the longest living stars in the universe, so life would likely have had a long time to take hold.

Unfortunately, we may never know much more about this intriguing new planet because it is so far away. At 500 light-years distance, with each light-year being almost 6 trillion miles, it is too far away for even next generation space telescopes to study in any detail. However, with red dwarfs accounting for about 85% of the stars in our galaxy, the odds of finding other Earth-like planets in the habitable zone closer to home are looking good.

Join the Springfield Stars Club on Tuesday, May 27th at 7:30pm at the Springfield Science Museum. Dave Kelly will talk on how to maintain your telescope and Alan Rifkin will talk on telescope choices for amateur astronomers. Dave Kelly is a telescope-maker and master lens-crafter, retired from Liebman Optics and Jena Optics. Alan Rifkin is president of the Springfield Stars Club and owner of FAR Laboratories, currently the only telescope dealership in Massachusetts. Refreshments will be served, and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per non-member.