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THE night sky is filled with stars you will never see with the naked eye. It鈥檚 not that they are too far away. In fact, they are some of our closest cosmic neighbours. Their lack of visibility is down to being cold and dim, at least as far as stars go. And yet they are anything but dull. Until recently, they held an astonishing secret.

In the quest to find life beyond our solar system, we have long sought the familiar: an Earth-like planet orbiting a sun-like star in the habitable zone or at just the right distance to have liquid water. Worlds around colder stars were not on the agenda. But then NASA鈥檚 Kepler space telescope spent the past decade snaring exoplanets. The haul revealed that most potentially habitable worlds, or at least the ones we are most likely to spot, exist in star systems quite unlike ours.

With Kepler due to run out of fuel at any moment, the next generation of exoplanet prospectors are shifting their focus to these worlds. Even as they do, however, planetary scientists are at odds over how habitable they are. One hallmark of colder stars is that their habitable zones are much closer in, which bestows some unfamiliar characteristics on the planets that reside there. Can life survive so close to a fiery ball of gas? And if so, what detectable signatures would it produce?

It has been more than two decades since we first glimpsed a world orbiting another star. One of the earliest finds was Pegasi 51b, discovered in 1995. Similar in size to Jupiter, it circles a sun-like star, albeit some 20 times closer in than Earth, a scorching feat that was considered impossible at the time.

When Kepler launched in March 2009, ground-based telescopes had detected just 300 or so exoplanets. It was designed to determine whether Earth-sized planets in habitable zones were common within the Milky Way. But along the way it has snared a multitude of exotic specimens, too (see 鈥�Worlds apart鈥�) .

Kepler searches a tiny patch of sky for telltale dips in the brightness of stars as planets pass in front of them, known as transit events. So far, it has found , with another 2000 or so awaiting verification. The finds are biased towards large planets closer in, because these cause the biggest dips (see diagram). Among them are all manner of 鈥�hot Jupiters鈥�, gas giants like Pegasi 51b that orbit closer to their stars than conventional models of planetary formation can explain; and mini-Neptunes, gaseous worlds that nevertheless might host planet-wide liquid water oceans.

Nothing generated as much excitement as the 30 鈥淓arth-like鈥� exoplanets Kepler has turned up. These worlds qualify for that label by virtue of being less than twice Earth鈥檚 size, and orbiting within their star鈥檚 habitable zone. Kepler 452b, announced in 2015, is considered the most Earth-like of the lot. At 1.6 times the size of our planet, it stands a good chance of being rocky. And its parent star is similar to our sun, albeit roughly 1.5 billion years older and thus slightly larger and brighter.

Even so, a true Earth twin 鈥� a rocky, Earth-sized planet orbiting in the habitable zone of a similarly aged, sun-like star 鈥� remains elusive.

鈥淲e鈥檙e left a bit in limbo,鈥� says , an astronomer at the University of Birmingham, UK. To answer the question of whether life exists elsewhere, he says, we do not need to find a second Earth in a carbon copy of our solar system. Indeed, Kepler observations suggest that most of the potentially habitable planets orbit stars that are much smaller and cooler than our sun, known as red or 鈥淢鈥� dwarfs.

What we need to do, Triaud argues, is refocus on these long-overlooked star systems: 鈥淚f you want an answer in our lifetimes, then M dwarfs are the only choice.鈥�

He may have a point. Earth twins could hardly be more difficult to find, never mind study. Any world in the habitable zone around a sun-like star would take roughly an Earth year to complete its orbit. That limits the number of transits to one per year, which makes spotting second Earths a long game.

Alien atmospheres

Planets around cool, dim stars are different. They can afford to orbit much closer in, so can zip around in as little as 10 days. In that case, we might be able to see as many as 15 transits every year, says Triaud. That would give planet hunters more data than ever 鈥� not only to confirm a planet鈥檚 existence and size, but also to investigate its atmosphere, where the best clues to habitability lurk.

Another thing M dwarfs have going for them is their sheer abundance. They make up about two-thirds of all stars, including most of our nearest neighbours, so we will never be short of targets. based on Kepler data suggest that between a quarter and a third of them host rocky worlds in the habitable zone, so they represent 鈥渢he biggest population of Earth-like planets鈥�, says Triaud.

Last year, Triaud was part of a team led by Micha毛l Gillon at the University of Liege in Belgium that announced the discovery of seven rocky planets orbiting an ultra-cool star called TRAPPIST-1. Three reside in the habitable zone. The find astonished astronomers, and not only because models had suggested that it should not be possible for this many planets to orbit so close to one another.

Adding to the surprise was the observation that their orbital periods are all near-whole-number ratios of each other. In the time it takes for the outermost planet to complete two orbits, the next one in has completed three, the one after that four, then six, nine, 15 and 24. That oddly regular pattern should shift the gravitational relationships between the planets until two or more occupy the same orbit, with obvious consequences 鈥� and yet they have not crashed into each other.

鈥淚t鈥檚 such a weird system,鈥� says at the SETI Institute in California, who was not involved in the work. It is weirdly promising, too. Not only are all seven made of rock, as opposed to ice or water, but the atmospheres of six of them are not dominated by hydrogen 鈥� similar, in that respect, to our own atmosphere.

Franck Selsis, an astronomer at the University of Bordeaux, France, who was also part of Gillon鈥檚 team, says future observations could better pin down the planets鈥� densities. That could show whether they are rich in volatile gases, such as methane. If they are, planets around M dwarfs will look even more auspicious as crucibles for life, he says.

Either way, TRAPPIST-1 should be the first surprise of many in the post-Kepler era. The latest exoplanet-hunting ventures are designed to survey M dwarfs. Perhaps the most high profile is NASA鈥檚 Transiting Exoplanet Survey Satellite. Launched in April, TESS is a space telescope 鈥� the successor to Kepler 鈥� that will search about 200,000 stars of all types for exoplanets. Not all of those stars will be M dwarfs, but many will be 鈥� and TESS鈥檚 instruments are tuned to their reddish light.

Then there is SPECULOOS, an Earth-based search for transit events around 500 ultra-cool M dwarfs. It has been up and running since December last year, and operates from an array of four telescopes in Chile鈥檚 Atacama desert. Triaud, who is part of the team, anticipates finding 鈥渁 handful or two鈥� of Earth-sized planets with atmospheres that we might be able to probe in the next five to 10 years.

Just finding out whether an exoplanet has an atmosphere is the trickiest bit. Instruments such as CARMENES at the Calar Alto Observatory in Spain, and SPIRou, part of the Canada-France-Hawaii telescope on Hawaii鈥檚 Big Island, can help. They are designed to detect wobbles in M dwarfs鈥� positions, caused by planets tugging on them as they orbit. From that we can calculate a planet鈥檚 mass, which we need in addition to its size to figure out its density. This in turn gives us a first hint as to whether it has an atmosphere 鈥� because if a planet appears to be large but not very dense, we assume that some of it is made up of gases rather than rock.

To really dive deep into the atmospheres of promising new worlds, however, we are going to need NASA鈥檚 James Webb Space Telescope, due to launch in 2021. in the atmospheres of many M dwarf planets. The race is on to find as many targets as possible to go sniffing around, whether those planets are conventionally Earth-like or not. 鈥淭here are very good reasons to look at M dwarfs,鈥� says at the University of Gottingen in Germany, who leads the CARMENES team. 鈥淣ow everyone is rushing at them.鈥�

鈥淥n 鈥榚yeball planets鈥�, life could lurk in a narrow band of perpetual twilight鈥�

Just as more planet hunters tilt at cooler stars, however, others are raising questions. Searching for habitable planets around M dwarfs might turn out to be easy, says at Harvard University, but these planets are likely to be very different from Earth 鈥� so much so that they may not be truly capable of supporting life.

For one thing, their close-in orbits bathe them in a sterilising cocktail of radiation and solar flares. Then there鈥檚 the fact that they are generally so close in as to be tidally locked, with one side constantly facing the star.

This happens because the gravitational tug of the star is slightly stronger on the planet鈥檚 near side than its far side. Over time, this forces the planet鈥檚 rotation to match its orbit, so its day in effect becomes its year. One side endures never-ending daylight, the other interminable night. The dayside would probably be too sun-baked to be hospitable, the nightside too cold. The best hope for life on these 鈥渆yeball worlds鈥�, assuming that bizarre weather patterns do not interfere, would lie in the razor-thin band where there is perpetual twilight.

So perhaps Earth-like planets around M dwarfs aren鈥檛 so hospitable after all. For Loeb, that means we should not abandon the search for Earth-like planets around sun-like stars. 鈥淔or life, I think we should start with what we know,鈥� he says. 鈥淓arth is the only place where we know life started.鈥�

Earlier this year, Loeb and his colleague Manasvi Lingam adapted a cost-benefit analysis method from economics to identify the best search targets, given constraints such as telescope sensitivity and factors that could make planets around M dwarf stars less habitable. The showed that stars of between 0.8 and 1.5 solar masses, or roughly comparable to the sun, are the best bet if you want to maximise the chance of detecting biosignatures 鈥� though that depends largely on how likely you think it is that M dwarf planets can host life, which is far from settled.

Advocates for M dwarfs are undeterred. They point to suggesting that dense atmospheres and planetary magnetic fields might offer some protection from solar winds and radiation. As for tidal locking, Triaud says we won鈥檛 know if it prohibits life until we can actually look at tidally locked planets. 鈥淢ost of the objections are theoretical at the moment. We need to make observations.鈥�

What those observations will reveal is anyone鈥檚 guess. If there鈥檚 a lesson from two decades of exoplanet hunting, it is that we should expect the unexpected. So even if a sizeable number of astronomers think there is every reason not to take M dwarfs seriously in the search for alien life, that is exactly why we should, says Marchis. 鈥淚 hope they鈥檙e going to let us do some very stupid things with those very expensive instruments, because that鈥檚 the way we鈥檙e going to find life.鈥�

This article appeared in print under the headline 鈥淩ed sun rising鈥�

• Didier Queloz will be speaking about exoplanets at 快猫短视频 Live, which is running in London from 20-23 September. For more information see