Astronomers have used an innovative technique to discover four super-Earth-size exoplanet candidates orbiting Tau Ceti, a Sun-like star 12 light-years away.

When exoplanets were first being discovered by the handful in the 1990s, teams competed to measure the wobbles of nearby stars, induced by the gravitational tugs of orbiting planets. A star’s radial velocity (its motion toward or away from Earth) can be measured by its spectrum, where the Doppler effect will shift spectral lines as the star wobbles. The tinier the wobble, the tinier the shift — and the tinier the planet doing the tugging.

Now astronomers are testing the limits of what this planet-finding method can achieve.

Initially, the process of exoplanet discovery was relatively straightforward, albeit time-consuming and telescope-intensive. A hot Jupiter on a tight orbit induces a large wobble in its star, shifting it back and forth several meters per second over a period of days, generating a clear, regular signal. But if astronomers want to detect an Earth-size planet at an Earth-like distance from its star, they’ll need far more sensitive radial velocity measurements — around 0.1 m/s. And things get tricky when astronomers begin reaching below 1 m/s. It’s easy to confuse the motions on a star’s surface for the motion of the star itself or with internal signals generated by the instrument itself. A small planet’s signal can become lost in the noise.

Fabo Feng (University of Hertfordshire, UK) and colleagues are taking a new approach to radial velocity measurements, digging deep into the noise to get at planet-induced signals. The team examined more than 9,000 spectroscopic measurements of the star collected using the High-Accuracy Radial Velocity Planet Searcher (HARPS) instrument, a spectrograph installed on the European Southern Observatory’s 3.6-meter telescope at La Silla Observatory in Chile.

When the team removed all known sources of noise from the star’s spectra, four regular signals remained. The planet candidates are named Tau Ceti e, f, g, and h. Two of the candidates had already been suspected in a previous study, while two others (g and h, with tight orbits of 20 and 49 days) are brand-new finds. The radial velocity these planets induce on their star is as low as 0.3 m/s — in other words, the instrument has almost, but not quite, reached the capability to find an Earth-size planet in an Earth-like orbit.

“We realized that we could see how the star's activity differed at different wavelengths and use that information to separate this activity from signals of planets,” said Mikko Tuomi (University of Hertfordshire, UK). “No matter how we look at the star, there seems to be at least four rocky planets orbiting it.”

The results will be published in the Astronomical Journal (full text here).

Paul Robertson (Penn State), who was not involved in the study, found the method intriguing. “Regardless of whether all four candidates stand the test of time, this work represents an important effort to advance our ability to distinguish bona fide planets from astrophysical and instrumental noise,” he says.

Cooler and smaller red dwarf stars have become the target of many new planet searches for their proximity to Earth, but such stars are prone to high-energy flares and microflares that could endanger life on surrounding planets. Tau Ceti is the nearest single Sun-like star, and it doesn’t exhibit much high-energy activity. However, Tau Ceti has 10 times the comets and asteroids found in our solar system, so the chance for impacts could be considerably higher for Tau Ceti planets than on Earth. Still, it remains a popular target for SETI searches.

Two of the star’s planets, Tau Ceti e and f, orbit in the habitable zone. That means that, if the planets had solid surfaces, they’re at the right distance from the star for liquid water to exist on their surfaces. The planets have masses at least 4 times Earth’s mass, so it’s unclear if they’re more like solid super-Earths or gaseous mini-Neptunes. The planet's sizes are still unknown, so future measurements will be key to determining the planets' compositions.