Astronomers have found a system of six super-Earths, one of which is at the "Goldilocks" distance for sustaining liquid water. The Sun-like host of the system lies only 40 light-years away.
“Are we alone?” To answer the biggest question of our time, first we have to start with the smaller ones: Can we find exoplanets that look like Earth? And when we find them, will they harbor the conditions for life?
Astronomers began detecting planets around other stars in 1989, with the latest exoplanet tally at 846. Most of the initial finds were Jupiter-mass or greater, but in recent years, new instruments, such as the High Accuracy Radial Velocity Planet Searcher (HARPS), have been designed to detect the signature of smaller planets. With that, astronomers are zeroing in on their prime target: a Sun-like star harboring an Earth-like planet in its habitable zone. At this “Goldilocks” distance, the planet is not too hot, not too cold, but has just the right temperature to harbor liquid water on its surface.
Now, astronomers have discovered one of the most promising candidates for an Earth-like twin. An international team led by Mikko Tuomi (University of Hertfordshire, England) and Guillem Anglada-Escudé (University of Göttingen, Germany) will publish the study in Astronomy & Astrophysics, claiming the detection of three additional planets around the dwarf star HD 40307, which lies just 40 light-years away.
In 2009, the HARPS team announced the existence of three super-Earths orbiting HD 40307. The planets each tugged on the star as they zipped around, producing a tell-tale wobble in the stellar spectrum. All three circled the star in tight orbits, well within a Mercury-like orbit, leaving them far too hot to sustain liquid water.
Tuomi and Anglada-Escudé re-analyzed the same publicly available data using a new technique, revealing three additional super-Earths around the same star. The team pioneered the technique to boost the sensitivity to smaller planets. They also avoided false signals induced by the star’s own activity by using only the reddest parts of the star’s spectrum.
The sixth, most distant planet HD 40307g is garnering the most interest. With a mass at least 7 times that of Earth, the planet could be rocky. And with a period 200 days long, the planet receives roughly 62% of the radiation the Earth receives from the Sun, putting it squarely in the habitable zone. If the planet is rocky — and that’s a big if — it could sport liquid water on its surface.
For those keeping track, HD 40307g isn’t the first planet to be detected in a star’s habitable zone. But most discoveries so far have had important caveats attached: some, such as 55 Cancri f, are as massive as Jupiter. Others, such as GJ581 and GJ667C, lie so close that their orbits become tidally locked — when one side of the planet always faces the star, the prospects for life become less likely. The most promising candidate to date has been Kepler-22b, but though this planet qualifies as a super-Earth in a habitable zone, it's unlikely to have a rocky composition.
One advantage HD 40307g has over other super-Earth competition is its long orbit. Since it’s farther away from its host star, its orbit is not tidally locked, so the day-night rotation could be similar to that of Earth. Another bonus point to life is the exoplanet’s host star: just like the Sun, it’s 4.5 billion years old, it’s similar in size, and it’s not magnetically active.
“The star HD 40307 is a perfectly quiet old dwarf star, so there is no reason why such a planet could not sustain an Earth-like climate,” Anglada-Escudé explains.
Additional observations are needed to confirm that the candidate is indeed an exoplanet. The authors caution that ground-based telescopes won’t be able to decipher much more in the way of details, such as the planet’s radius. The planet’s orbit probably doesn’t cross the star from our point of view, so transit observations such as those conducted by Kepler are impossible.
Instead, the authors suggest that the planet — separated from its host star by 40 milli-arcseconds — could make a good observing target for future telescopes designed to image exoplanets directly. (However, funding for such telescopes is currently nonexistent.)
While the search for Earth's twin continues, HD 40307g takes astronomers a little closer to their target.