Monday, July 27, 2015

Can Earth’s bigger, older cousin sustain life as we know it?

The light from the star Kepler 452 dimmed just a fraction. Then, 385 days later, it happened again. Astronomers now know that a planetonly a little bigger than the Earthis causing the dimming, as it blocks some of the light heading our way. It is orbiting a sun-like star and is the closest analogue to Earth discovered yet.

This way of detecting planets is called ‘Transit Method’ and has turned out to be one of the most successful ways of identifying other worlds. So far, astronomers have detected 5,583 planetary candidates around stars other than our Sun, with 1,879 already confirmed.

Wait. Take a deep breath.

Now imagine almost 1,900 confirmed planets outside our solar system. To put this in perspective, throughout the entire history of humanity, say the last two decades, we knew of planets only in our own solar system – we even managed to demote one of them! It was only in 1995, when astronomers confirmed the existence of the first extra-solar planet – 51 Pegasi and the number of planets is now steadily increasing.

Most of the planets discovered so far are much bigger than the Earth and often lie quite close to their parent star; much of this is a selection boa as detection techniques are better in detecting these kinds of planetary systems. But of course, we want to find Earth-like planets – small, rocky worlds, orbiting sun-like stars at a distance where water can exist in liquid form. This last bit is potentially important for life. Neither too hot, nor too cold. This is called the Goldilocks zone or more formally, the Habitable Zone. In our own solar system, Venus is too close to the Sun and Mars is just a little too far. But Earth is in the middle of the habitable zone and has ably supported life for the past four billions years.

If we can find earth-sized planets in their respective habitable zones, the thinking goes that perhaps these places tend to originate life as well. Furthermore, on at least some of these worlds, biological evolution would have led to the development of complex organisms as well.

But wait, one step at a time.

First we have to detect Earth-sized planets in habitable zones. In 2011, astronomers discovered an earth-sized planet, Kepler 20e. However, its orbit was only six days long and therefore too close to the Sun. The same year came the discovery of Kepler 22b. This time, the planet was in the habitable zone of a sun-like star, but it was double the size of the Earth and it is quite likely that it is made of predominantly gases like Jupiter and other big planets in our solar system. Later, in April of last year, astronomers discovered Kepler 186f, an Earth-sized planet in the habitable zone. It is a promising candidate but with just one potential catch – it orbits a star that is smaller and dimmer than our Sun. Its habitable zone, therefore, is closer to its star. I think this is a great candidate for a planet that may host life. The only thing is that it doesn’t orbit a sun-like star.

That brings us to Kepler 452b. This planet is 60 per cent bigger than the Earth. It orbits a sun-like star and it takes 385 days to go around its star. Astronomers believe that it has an atmosphere thicker than the Earth and it also has an active volcanism on its surface. However, it is located 1,400 light years away. Even if we were to find a way to travel fast, this will still be a little too far away.

An artist conception of Kepler 452b (Credits: NASA Ames/JPL-Caltech/T. Pyle)

In addition, the planet is 1.5 billion years older than the Earth. This can be both good and bad. This older age gives the planet plenty of time for life to develop. On Earth, life started early, but then it took several billion years to develop complex species like the turtles, the elephants, and the species that are looking for life on other planets. Just because it developed this way on Earth, this has no guarantee that it will happen the same way on another planet. But having more time – 1.5 billion more years – nevertheless is probably good when it comes to possible diversity of life.

On the other hand, the central star of Kepler 452b is also 1.5 billion years older, and that means that it’s also a bit brighter than our Sun. Stars like our Sun, brighten up a little as they age and that can have potentially devastating impact on the habitability of the planets. Our own Sun will get 10 per cent brighter in the next billion years or so, and this extra heat will probably result in the evaporation of oceans on Earth making our planet inhospitable to life as it exists today. It is impossible to predict the future of humanity – or what our descendants will be called – that far into the future. Nevertheless, relocation will be the only option for survival, if they still reside on our planet. However, we don’t know for sure if the slightly larger size and being slightly farther away from the Sun will give Kepler 452b some extra time for habitability or not.

Kepler 452b is a great candidate for life. But hold the celebrations, astronomers estimate that 10 per cent of stars in our galaxy host Earth-sized planets that may exist in the habitable zone. In a galaxy of 200 billion stars, that leaves us with 20 billion potentially habitable planets! I am quite sure, no, I am certain that within the next few years, we will find more promising candidates much closer to home. Furthermore, I am quite sure that on at least one of these worlds, we will detect an atmosphere that has been transformed by the existence of life on that planet.

Now that will be something worth celebrating. Stay tuned.

from The Express Tribune Blog

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