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The hunt to learn more about our nearest exoplanet, the Earth-like Proxima b, has come up empty-handed. If the planet ever passes between its star and us, we could rapidly learn a lot about it – but observations show it is unlikely to do so.

When Proxima b was discovered orbiting our closest neighbouring star, Proxima Centauri, two months ago, its mass, which is similar to that of Earth, as well as its location in the habitable zone, created excitement about the possibility of it harbouring life.

The best way to learn more about an exoplanet is to watch it pass between its star and our telescopes, which can yield a wealth of knowledge about the planet’s size, atmosphere and even its surface conditions. Such a transit so close to home would be a monumental result.

Before Proxima b was spotted, David Kipping at Columbia University in New York and his colleagues had spent over 40 days in 2014 and 2015 hunting for planets orbiting Proxima Centauri using the MOST Space Telescope. Once the discovery was announced, they narrowed their search, looking for signals that could match a planetary transit.

“It almost felt like too much to ask for, because it’s such an incredible planet: it’s an Earth-mass planet, at the right distance for life, around the nearest star,” says Kipping. “It’s already checking three boxes that you could optimally hope for in any exoplanet discovery, and we were hoping that it also transits.”

Long shot

One problem with the hunt was that Proxima Centauri’s relative faintness and constant flares made it difficult to see if the planet passed in front of the star. In addition, the likelihood of a transit – based on the geometry of the planet’s orbit – had been calculated at only 1.5 per cent.

In the end, Kipping’s team did not see Proxima b. They did observe one potential signal, but comparison with data from the ground-based HATSouth telescope network indicated it was unlikely to be a transit.

“Given how active the star is, given the low chance of this actually transiting and given the fact that we don’t see it in the ground-based data, my feeling is that Proxima b is not a transiting planet,” says Kipping.

If further observations confirm this, we will have a while to wait to learn more about Proxima b. Without a transit, we will instead have to observe the exoplanet directly to learn more about it. The powerful James Webb Space Telescope, set to launch in 2018, will be able to supply some details about Proxima b’s atmosphere, but more detailed information is likely to be years or even decades away with the next generation of telescopes.