Astronomers have discovered a new candidate planet around Proxima Centauri, the nearest star to the Sun at 4.2 light-years. The find brings the total number of candidate planets in this system to three.

The brave new world, dubbed Proxima d, wouldn’t make a pleasant place to live. Its orbit takes it around its faint red star in just five days, close enough that liquid water probably doesn’t exist on its surface. But the method by which the astronomers detected Proxima d, by measuring the wobble of its host star, might well lead to the discovery of more habitable worlds.

From Wobbles to Planets

As a planet orbits a star, it appears at first glance to orbit the star’s center. But two objects always orbit their center of mass, and even a small planet around a star has enough mass to offset that center, such that the star wobbles as its planet goes round. Astronomers can spot that wobble when they measure a star’s radial velocity, its motion toward or away from Earth.

A more massive and/or closer-in object will cause its star to sway even more. The hot Jupiter 51 Pegasi b, which circles around every four days, creates an easily observable shift in its host star of 70 meters per second. Earth, on the other hand, induces a motion of only 0.1 meters per second (10 cm/s) on the Sun.

Getting to Earth-like planets around Sun-like stars seems a matter of improving the precision of measurements. But stars can fool those who would hunt planets. When a star’s boiling surface generates starspots, plumes, and prominences, that activity can produce radial-velocity signatures in the realm of meters per second. An Earth-like signal can easily get lost in the mix.

João Faria (University of Porto, Portugal) and colleagues challenged that limit when they found Proxima d, as announced in Astronomy & Astrophysics. The candidate planet wobbles its star by just 40 cm/s. That detection was possible both because of a new instrument — the ESPRESSO spectrograph on the Very Large Telescope in Chile — and a keen eye on the star’s activity.

ESPRESSO takes in the light coming from VLT’s four Unit Telescopes, each 8.2 meters in size, and splits it into a spectrum. The result is impressively high resolution, with the capability of detecting an Earth-like planet around a low-mass star, as well as long-term stability in the measurements.

In addition to detecting the slight shift in spectral lines as the star wobbles toward and away from Earth, the spectrograph can also detect emission lines indicative of stellar activity. None of these indicators had the same five-day signal as the candidate planet. However, while the team is confident of the discovery, the unknown unknowns led the researchers to label the planet a candidate.

“We performed several tests to rule out the possibility of stellar activity as the cause of this signal and they all pointed to a planetary origin being the most likely,” Faria says. “However, there is always a chance that we were fooled by the star.”

Proxima d as Herald of the Future

“Because this is a significant object (planet orbiting the nearest star to the Sun), the authors are cautious,” says Guillem Anglada-Escudé (Institute of Space Sciences, Spain), who led the discovery of the first confirmed planet, Proxima b. “But let me point out that the presence of such a planet is not a surprising result, but a beautiful discovery.”

Because we don’t know the angle at which it’s orbiting its star, Faria’s team can only estimate the planet’s minimum mass: it’s at least a quarter of Earth’s mass, or about twice the mass of Mars. Such a small planet produces a strong radial-velocity signature because it’s so close to its host: 3% of the average distance between the Sun and Earth, 10 times closer in than Mercury’s orbit around the Sun.

Proxima Centauri is no Sun — it’s a faint red star that radiates with 0.2% of the Sun’s brightness — so the planet isn’t quite as scorched as Mercury is. However, even if we assume Proxima d is an airless rock, with no atmosphere to trap heat as Venus does, it would still have an equilibrium temperature of 365K (200ºF), close to water’s boiling point. Life as we know it can’t exist there. Still, the world might hold other surprises. “The presence of an atmosphere is certainly a possibility,” Faria says.

The detection of Proxima d is an indication of what kinds of worlds ESPRESSO is capable of finding. “I would say that for orbital periods shorter than the rotation of the star (few tens of days), Earth-mass planets can now be systematically detected with ESPRESSO,” says Anglada- Escudé. “Until now, the measurement uncertainty was comparable to the stellar noise, and it was not possible to distinguish with enough precision. ESPRESSO is changing that.”

So, what of Proxima c? This candidate world, though the second to be discovered, is farther out than Proxima d and takes a full five years to complete an orbit. The data ESPRESSO has collected so far covers less than half that span, so it isn’t capable yet of seeing this more distant world’s signal. But ESPRESSO will continue observing our nearest stellar neighbor. Besides Proxima c, who knows what more worlds this system might hold?

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