“In space there are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours.”
- Giordano Bruno, 1584
In recent years, many planets have been found within the habitable zone - the region around a star where the temperatures are just right for a planet to sustain liquid water on its surface. By analysing data from NASA’s Kepler space telescope, a team of astronomers have announced the discovery of the most Earth-like planet yet detected. Kepler is a planet-hunting telescope that measures subtle changes in the brightness of stars to see if an orbiting planet is crossing in front of a star. This newfound planet, dubbed Kepler-186f, is the first confirmed Earth-sized planet in the habitable zone of another star. “This is the first definitive Earth-sized planet found in the habitable zone around another star,” said Elisa Quintana, research scientist at the SETI Institute at NASA’s Ames Research Center, and lead author of the paper published on April 17 in the journal Science.
Figure 1: Artist’s depiction of Kepler-186f, the first validated Earth-sized planet to orbit in the habitable zone of another star. Image credit: NASA Ames/SETI Institute/JPL-Caltech.
Before the announcement of Kepler-186f, the smallest habitable zone planets known are all somewhat larger than the Earth, placing them in the super-Earth-sized rather than Earth-sized regime. Although these planets are still potentially habitable, their environments are likely to be quite different compared to the Earth. Examples of these habitable zone super-Earths include Kepler-62e, Kepler-62f and Kepler-22b. Furthermore, Kepler has also detected a number of planets the size of Earth or smaller around other stars. However, these planets all orbit too close to their host stars and are therefore too hot to be habitable. Examples of these close-in planets include Kepler-20e, Kepler-20f, Kepler-78b and Kepler-37c. Unlike these planets which are either ‘at the right distance but too large’ or ‘at the right size but too close’, Kepler-186f is the first confirmed planet that has both the right size and the right distance.
Kepler-186f is part of a planetary system with 4 other known planets. The 4 companion planets, Kepler-186b, Kepler-186c, Kepler-186d and Kepler-186e, circle around their host star every 3.89, 7.27, 13.3 and 22.4 days. All 4 planets orbit much closer-in than Kepler-186f and are therefore too hot to be habitable. These 4 planets range in size from 1.07 to 1.40 Earth-radius and were confirmed using the first 2 years of data collected by Kepler, while Kepler-186f, the fifth planet, required an additional year of data.
Kepler-186f measures only 10 percent larger than Earth (1.11 ± 0.14 Earth-radius), making it a truly Earth-sized planet. Its host star is a red dwarf star about half the size of Earth’s Sun and located about 500 light years from Earth. Kepler-186f orbits within the habitable zone of its host star - the “Goldilocks” zone that is neither too hot nor too cold for liquid water to exist if Kepler-186f has an Earth-like atmosphere. Being a red dwarf star, the host star of Kepler-186f is cooler and dimmer than the Sun. It means that the habitable zone around the host star of Kepler-186f is located much closer-in compared to the habitable zone around the Sun.
Figure 2: The diagram compares the planets of our inner solar system to the planetary system hosting Kepler-186f. The parent star of Kepler-186f is a red dwarf star with half the size and mass of the Sun. Image credit: NASA Ames/SETI Institute/JPL-Caltech.
Figure 3: The 5 planets of the Kepler-186 planetary system compared to the inner planets of our solar system. Image credit: NASA.
Kepler-186f circles its host star in the habitable zone once every 130 days. The orbit of Kepler-186f around its host star is actually slightly smaller than the orbit of Mercury around the Sun in our own solar system. Kepler-186f’s orbit places it near the cooler, outer edge of the habitable zone. Despite receiving only 32 percent of the intensity of stellar radiation that Earth receives from the Sun, Kepler-186f is in fact comfortably within the habitable zone. This is because light from a red dwarf star is “redder” than light from stars like the Sun and it changes how an Earth-like planet would interact with the star’s light.
Compared to the Sun, whose dominant form of radiation is in the visible wavelength, red dwarf stars are cooler and a larger proportion of their energy output is in the form of infrared radiation. For Earth-like planets around red dwarf stars, infrared radiation is absorbed by ice instead of being reflected. Additionally, water vapour and carbon dioxide also absorb and trap infrared radiation. These characteristics make Kepler-186f more efficient at absorbing energy from its host star to avoid freezing over. As a result, Kepler-186f is still considered habitable even though it receives less light from its host star than Mars receives from the Sun.
Figure 4: Position of Kepler-186f within the habitable zone, show in comparison with a number of other known planets. Image credit: NASA/Chester Harman.
Although the size of Kepler-186f is known, its mass is not known since data from Kepler is unable to yield such measurements. In spite of this, Kepler-186f is small enough for its composition and mass to be well constrained using existing planetary models. These models predict that planets smaller than 1.5 times the size of Earth are unlikely to be dominated by hydrogen-helium gas envelopes like Jupiter or Neptune in our own solar system. Mass estimates for Kepler-186f range from 0.32 Earth-mass for a pure water/ice composition to 3.77 Earth-mass for a pure iron composition. The composition of Kepler-186f is highly unlikely to be anywhere close to these two extremes. Instead, Kepler-186f probably lies somewhere in the middle, most likely with a rocky composition similar to Earth. For an Earth-like composition, Kepler-186f would have a mass of 1.44 Earth-mass.
Red dwarf stars, like the host star of Kepler-186f, are by far the most common type of star in the galaxy. Such stars make up over 80 percent of the closest stars to the Sun. They are less massive, smaller, cooler and dimmer than the Sun, and they range in size from around 10 to 50 percent the size of the Sun. Besides being the most abundant type of stars, red dwarfs stars are good targets in the search for transiting habitable Earth-sized planets. This is because a red dwarf star is smaller than a Sun-like star, so an Earth-sized planet around a red dwarf star would have a larger planet-to-star size ratio. As a consequence, an Earth-sized planet transiting a red dwarf star would create a deeper transit than if the same planet were to transit a larger Sun-like star. Deeper transits are easier to detect than shallower ones.
Figure 5: Diagram showing the deeper transit depth of an Earth-sized planet transiting a red dwarf star. Image credit: NASA.
Furthermore, transiting planets in the habitable zone of red dwarf stars would undergo more frequent transits than those in the habitable zone of Sun-like stars. This is because the habitable zone of red dwarf stars is much closer-in, resulting in shorter orbital periods than those around Sun-like stars. Given the abundance of red dwarf stars, planets such as Kepler-186f are almost certainly the most abundant type of habitable planet. Unfortunately, Kepler-186f itself is just too far away for even future NASA missions, like the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST) to determine its composition and atmosphere. Nevertheless, TESS and JWST will be able to detect and characterize Earth-sized planets around the nearest stars. Most of these planets would be around red dwarf stars, much like Kepler-186f.
The discovery of Kepler-186f supports the emerging view that Earth may not be such a unique place, and that the galaxy is home to billions and billions of habitable worlds. “The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth,” said Paul Hertz, NASA’s Astrophysics Division director at the agency’s headquarters in Washington. “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.”
Elisa V. Quintana et al., “An Earth-Sized Planet in the Habitable Zone of a Cool Star”, Science 17 April 2014: Vol. 344 no. 6181 pp. 277-280.