It’s been a good week for new worlds. At a conference in the UK where astronomers brought out their new planets for show and tell, the “star” attraction is Gliese 581e.

That’s because it is the lightest exoplanet yet, barely twice the mass of earth, orbiting a red dwarf sun in a mere 3.15 earth days, and only 20.5 light years away.

The real message was that if planet hunters can find worlds that are just above earth size in either dimensions or density they will find true twin earths in the near future.

And for those that dream in troubled times of some fabled twin earth, Gliese 581e definitely isn’t the answer.

In a galaxy not really so far away … Gliese d

When you are as close to a “dim” red dwarf star as this planet is, tin turns to gas. And this type of sun, which is very common in the cosmic neighbourhood, spits off flares of matter and streams of ionising radiation of lethal intensity worse than those ejected by yellow stars like our sun.

But the star catalogued only as Gliese 581 had already been found to have three other planets, and just as the latest of them Gliese e, was found, another, Gliese d, which was discovered in 2007 was found to be a candidate for the first “waterworld”.

Astronomers have surmised for decades that super-sized earth like planets covered with oceans and perhaps continents of ice, could exist at certain distances from different types of stars.

This week, in the turbulent game of claim and counter claim in exoplanetary science, Gliese d looks like a waterworld, because it appears to have an atmosphere capable of retaining enough heat through a natural greenhouse effect to keep its surface above freezing point.

And it would be a much nicer place for a swim than a crawl, since at 7.5 times the mass of Earth, Gliese d has a crushing surface gravitational drag that would make it no place for upright bipeds with a lung heart systems like ours even if the air was rich in oxygen.

Which raises the issue, what are the 346 new worlds that astronomers have discovered and verified in as little as 14 years of accurate planet hunting like?

Almost all of them are unlike the major planets of our solar system, although often it is very difficult to decide exactly what they are like in even broad detail.

About half of them skim much closer to their suns than the innermost planet Mercury in our system. And most of those appeared to resemble “gas giants” like Jupiter or Saturn, and that didn’t make sense either.

How could they survive intact? The answer lies in their intense gravitational fields.

Take Gliese 436 b discovered in 2007, another star in the Gliese catalogue of hundreds of “nearer” stellar neighbours.

It had a mass and diameter a little larger than Neptune, yet scorched its way around its sun at a distance 15-times closer than Mercury orbits our sun.

Gliese 436 b, with an average surface temperature of more than 400 C, seemed very strange. One model suggests it is largely made of hot ice, or ice that is prevented from exploding into a cloud of steam by its own massive gravity. Another model says it is made of rocks surrounded by hot, dense layers of hydrogen and helium, trapped in the same gravitational grip.

Yet other exoplanets have been found with more ‘normal’ orbits by our standards, just not yet in exactly the right place or of the right size to resemble Earth.

Those discoveries are expected to come thick and fast in the next five years as new powerful space telescopes dedicated to planet hunting are placed in orbits where the basics of trigonometry and incredibly accurate and sensitive instruments can be combined to resolve earth sized planets in a largely automated mapping process.

Once these planets are catalogued they can also have their atmospheres analysed by spectroscopes and other means to find traces of water vapour, methane, oxygen and other indicators of biological activity as we know it.

If we find ozone destroying hydrofluorocarbons or other synthetic halons produced only on earth by industry we’ll know we are onto something “really” interesting.

Peter Fray

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