Astrophile: an alien sunrise just like Earth's

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: the star HIP 56948
Size: same as the sun
Temperature: same as the sun
Composition: same as the sun
Planets: same as the sun?

On an alien world remarkably like Earth, a sun rises. Its tiny disk scours the landscape with harsh, blue-tinted light... wait a minute, that's not right. On an alien world remarkably like Earth, a sun rises. Its vast face floods the landscape with an eerie blood red glow. No no ? this is still no place like home.

In the search for other Earths, the main goal is to find a planet the same size as ours that sits in the habitable zone ? the region around a given star where planetary surface temperature would be similar to ours, allowing liquid water to exist. Almost all stars have such a zone. Around a hot bright star it is further out than Earth's own orbit; with a dim star it is further in.

But while an Earth-sized world in one of these habitable zones might have seas and rivers, it would look quite different bathed in blue-white or red light. That could affect the development of life. To exploit the available light, plant leaves could be yellow, orange or red, according to research in 2007 by the Virtual Planetary Laboratory at the California Institute of Technology.

So some planet searches concentrate on stars that are roughly the same size, brightness and colour as our sun. But even these stars tend to have different chemical compositions, suggesting that their planetary systems may be different too.

Perhaps the best place to look for another Earth would be in orbit around another sun.

Step up, HIP 56948. Around 200 light years away, this star took the title of most eerily similar solar twin in 2007, deposing the previous lookalike champion, 18 Scorpii. New observations show that HIP 56948 is even more of a doppelganger than previously thought, and improve the odds of Earth-like habitats.

The latest precise observations can detect no difference in size, mass and brightness between HIP 56948 and the sun, and it is only about 17?C warmer (while 18 Scorpii is 54?C warmer).

Better still, while 18 Scorpii has appreciably more iron in its atmosphere, the composition of HIP 56948 is almost identical to the sun's.

That is promising. The Sun is depleted in some heavy elements compared with most other stars its size. One theory is that those heavy elements have been locked up in rocky planets instead of mixed up in the star.

If so, the authors calculate, HIP 56948's planetary system should have the same amount of Earth-like material as ours. "It is a hint that what is missing is material that could form other Earths," says team leader Jorge Melendez at the University of Sao Paolo in Brazil.

Because the chemistry is so similar, such planets are more likely to have similar minerals and rocks to Earth and the other terrestrial planets of the solar system.

The team have also found that this system has no giant inner planets that would destabilise an Earth-like world.

So perhaps when the sun rises on a planet orbiting HIP 56948, it casts a familiar, warming yellowish-while light ? a light that falls on blue seas, green leaves and granite mountains.

The light would even be familiar to Earthly artists, although this world perhaps has not had time to evolve its Raphaels and Turners. HIP 56948's only distinct feature is that it's about a billion years younger than the sun. If evolution is running along the same path, the authors say, "complex life may be just developing ... in any hypothetical Earth that this star may host".

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