IN 1990 Voyager took a photograph of Earth that was striking precisely because it showed so little. The spacecraft was six billion kilometres away at the time and the image it sent back was memorably described by Carl Sagan as a “pale blue dot”. Imagine, then, how pale such a dot would be if the planet in the picture were 113,000 billion kilometres away. Yet this is the distance to the nearest confirmed exoplanet—a planet orbiting a star other than the sun. That gives some idea of the task faced by those who study these bodies. Only in the most special of circumstances can they actually see their quarry. Mostly, they have to work with indirect measurements, like watching for slight dips in the intensity of a star’s light when a planet passes in front of it, a phenomenon known as a transit.

But if indirect observation is all that is on offer, then astronomers must make the best of it. And, as numerous presentations to a meeting of the American Astronomical Society held in Seattle this week show, they have both done so, and have plans to do better in future.

The most successful planet-hunting mission so far has been Kepler, a satellite launched in 2009 by NASA, America’s space agency, which collected data using the transit method until 2013, when a mechanical failure disabled it. It has since been revived, but has only recently begun transmitting data. However, combing of the data it collected in its first incarnation continues, and Douglas Caldwell of the SETI Institute, in Mountain View, California, who is one of the mission’s chief scientists, announced to the meeting the discovery of eight new planets. Three of these lie in their solar systems’ habitable zones (that is, they are at a distance from their parent stars which makes them warm enough for water on their surfaces to be liquid, but cool enough for it not to be steam). One of these three, known as Kepler 438b, is thought particularly Earthlike. It is a bit bigger and a bit warmer than Earth, but is probably rocky. It is therefore likely to be the subject of intense future scrutiny.

The pressure’s on

New ways of scrutinising exoplanets is a particular interest of Victoria Meadows, who works at the University of Washington. She explained to the meeting how an exoplanet’s atmospheric pressure might be measured. Only if that pressure is high enough could liquid water exist on a planet’s surface. Since, based on the lone example available, it looks likely that liquid water is a prerequisite for life, knowing a planet’s atmospheric pressures would help focus the search for a second.

Dr Meadows proposes to make the measurement by looking at a planet’s atmospheric oxygen. This gas consists mainly of diatomic molecules (O2), with a sprinkling of triatomic ones, known as ozone. But it also contains O2 dimers. These are associations between pairs of molecules, which are not bound together strongly enough to count as molecules in their own right. The number of dimers depends on the atmospheric pressure. The higher this is, the more dimers are created by oxygen molecules being squeezed together. Since molecules and dimers absorb different frequencies of light, studying the spectrum of starlight that has passed through a planetary atmosphere could reveal its atmospheric pressure. Sadly, no existing instrument is sensitive enough to use Dr Meadows’s technique in this way. But this should change in 2018 when, if things go according to plan, the James Webb space telescope will be launched by NASA.

Dr Meadows is also interested in planetary water itself and particularly in a technique which could allow oceans on other worlds to be detected directly. An ocean acts as a mirror. And the bright reflection from such a mirror would, if it could be seen, give that ocean’s existence away.

The optimum moment for seeing it is when, as observed from Earth, a planet’s parent star is more or less behind the planet in question. Then, starlight glancing off an ocean on the planet’s limb will produce a glint that the same light glancing off a dull, rocky surface would not. That glint would give the ocean’s existence away.

This trick has actually been tried, using a probe bound for the moon to observe glints from Earth’s oceans. The results of this experiment have let Dr Meadows calibrate her expectations about what might be visible from exoplanets. Her conclusion is that it would be possible, using an instrument called Exo-C that is now on NASA’s drawing board, to see glints from planets going around nearby stars.

Whether there will actually be any oceans for such a mission to detect was the subject of a talk by Laura Schaefer of the Harvard-Smithsonian Centre for Astrophysics. Ms Schaefer has built a computer model that tries to show how fast oceans will develop on different sorts of planet.

This is not just a question of how much H 2 O a planet harbours. On Earth, for example, 50-90% of the stuff is tied up in minerals and is thus unavailable to float on the surface. These minerals are, however, heated, kneaded and moved around by plate tectonics—a process that sometimes liberates their aqueous content. Ms Schaefer assumed something similar goes on in rocky exoplanets (a brave assumption, since there is little sign of plate tectonics on Mercury, Venus or Mars), and therefore put into her model what information is known about this grand hydrologic cycle.

The great ocean of truth

She let the planets in the model evolve for ten billion years (the typical lifetime of the kind of star most planets orbit), and saw that they do indeed develop oceans, although on different schedules. An Earth-sized planet gets deep oceans quite quickly—within 500m years—but these then become steadily shallower. A planet with five times Earth’s mass takes about a billion years to form a shallow ocean, which then increases steadily in depth.

If oceans are crucial for life it looks, on the basis of Ms Schaefer’s model, as though biology could get a head start on an Earth-sized planet—if, as the model assumes, that planet has the same chemical composition as Earth. Finding what a planet’s rocks are made of is harder than analysing its air. But a couple of unusual cases are, as it were, illuminating the question.

Two known exoplanets orbit their stars so closely that their rocky surfaces are evaporating. Both thus sport dusty tails. These are something that can be examined telescopically, and compared with models of what they might contain. In a paper published last month Rik van Lieshout of the University of Amsterdam did just that. He modelled the kinds of tails that would be created if the dust from these two planets consisted of particular known minerals. His conclusion is that their tails could be composed of either of iron-rich silicates or of corundum—a substance that, when of gem quality, is better known as sapphire. Not pale blue dots, then, to be sure. But possibly pale blue streaks.