Billions of Earth-like planets may orbit red dwarf stars in our Galaxy
A new study suggests there could be billions of potentially habitable Earth-sized planets orbiting red dwarf stars in our Galaxy, including one just 13 light years away.
Researchers at the Harvard-Smithsonian Center for Astrophysics (CfA) have been studying data captured by NASA's Kepler telescope and examining planet candidates orbiting red dwarfs.
Scientists estimate there are at least 75 billion red dwarf stars in the Milky Way, making up three quarters of all the stars in our Galaxy.
The study, led by Courtney Dressing, looked at 95 planet candidates (which require further observation before they can be confirmed as planets) orbiting 64 red dwarf stars. The research found 3 planet candidates were similar in size to Earth and orbited their parent star at a distance where the temperature would allow water to exist on the surface, leading to a statistical conclusion that 6 per cent of red dwarf stars have Earth-sized planets in the habitable zone.
Using the 75 billion figure as the total estimated number of red dwarfs in our Galaxy, the research suggests there could be 4.5 billion Earth-like planets orbiting red dwarf stars in the Milky Way.
"Now we realize another Earth is probably in our own backyard, waiting to be spotted"
Red dwarfs are cooler and smaller stars than the Sun and are not visible to the naked eye. An average red dwarf star is about one-third the size of our Sun and only one-thousandth the brightness.
The not too hot not too cold habitable zone of red dwarfs is closer to the star because they are not as hot. Also, because red dwarf stars are not as bright and planets orbiting them are relatively larger, they will be easier to spot as they transit the face of the star.
Lead author Courtney Dressing commented: "We thought we would have to search vast distances to find an Earth-like planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted."
Of the 95 planet candidates studied, others were not the right size or temperature to be considered Earth-like, as shown in the diagram below.
Co-author of the paper David Charbonneau said of the findings: “We now know the rate of occurrence of habitable planets around the most common stars in our galaxy. That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought.”
By analyzing publicly available Kepler data, CfA astronomers identified 95 planetary candidates circling red dwarf stars. Of those, three orbit within the habitable zone (marked in green) - the distance at which they should be warm enough to host liquid water on the surface. Those three planetary candidates (marked with blue dots) are 0.9, 1.4, and 1.7 times the size of Earth. In this graph, light received by the planet increases from left to right, and therefore distance to the star decreases from left to right. Planet size increases from bottom to top. Credit: C. Dressing (CfA)
Natalie Batalha, Kepler mission scientist at NASA's Ames Research Center said: "We don't know if life could exist on a planet orbiting a red dwarf, but the findings pique my curiosity and leave me wondering if the cosmic cradles of life are more diverse than we humans have imagined."
NASA's Kepler space telescope launched in 2009 to search for extrasolar planets, also known as exoplanets. Kepler is monitoring over 150,000 stars for dips in light that could be caused by a planet crossing in front of the star. So far over 800 planets have been confirmed, with several thousand more planet candidates awaiting further observation. In 2012 the Kepler mission was extended by NASA until 2016.
Last year astronomers discovered a planet orbiting our nearest star system, Alpha Centauri, which is just 4.3 light years away. However, although the planet has about the same mass as Earth there is no chance of life as it orbits Alpha Centauri B at a distance of just 6 million kilometres, much closer than Mercury to the Sun.
The results of the red dwarf study are published in The Astrophysical Journal.