Monday, April 21, 2014

Kepler-186 f: An Earth-size planet in the Habitable Zone

“Are we alone in the Universe”? This is one of the greatest question mankind has been asking for a millennia. Now, we are a step closer to answer that question. NASA’s Kepler space telescope, launched in 2009, has discovered an Earth-size planet in the “Habitable Zone” of a dim, cool star called “Kepler-186” nearly 500 light-years from us. 

An artist’s imagination of the Earth-size planet, Kepler-186f, and four other planets in this system. The star (Kepler-186) is a cool, low mass star with roughly half   the size of our Sun. This system is about 500 light-years from us. Kepler-186f is the smallest Earth-size extrasolar planet found to be in the Habitable Zone to date. Credit: NASA/Ames.

The planet, named “Kepler-186f” (because it is the fifth planet in this system starting with the first discovered planet with letter “b”), is only 11% larger than the Earth. To get a sense of how close to Earth-size this planet is, assume you are 6 feet tall. And now imagine a friend  of your’s who is 6.6 feet tall ( a typical basketball player height). Not too much of a difference, right?

Kepler-186f orbits its star every 130 days. That means, a “year” on this planet is about roughly a third of the Earth’s year!  This 130 day orbit around its star puts Kepler-186f right in the so-called “Habitable Zone”. What is a “Habitable Zone”? It is a region around a star where a rocky planet (like our Earth or Kepler-186f), with proper atmosphere, can have liquid water on its surface. If a planet is too close to the star, it would be too hot to have liquid water on its surface. If it is too far from the star, it would be too cold.   Kepler-186f  has the right size, and is at the right distance from the star. It just needs to have the "right" kind of the atmosphere to have water on it's surface.

The sizes of our Earth and Kepler-186f (inset), and the sizes of the “Habitable Zones” for both Solar System and Kepler-186 system. The star Kepler-186 is a cool,  low mass star (and hence appears “red” or “orange”). So the “Habitable Zone” is closer to the star compared to our Solar System (just like you have to be closer to a camp fire if the fire is not hot enough). Credit: NASA/Ames

The most exciting part of the discovery of  Kepler-186f is that,  it is the smallest Earth-size planet found to be in the Habitable Zone of any star other than our Sun! But is it “Habitable”? There is a big difference between being in the “Habitable Zone” and being “Habitable”. Earth is in the Sun’s “Habitable Zone” and is “Habitable” (i.e., it has a  life). Mars is also  in the Sun’s “Habitable Zone”, but so far we found it to be not “Habitable” (i.e, no life found on Mars…yet). So just because a planet is in the “Habitable Zone” doesn’t mean it is “Habitable”. We do not know if Kepler-186f is a “Habitable” planet. We can only speculate. And there appears to be a lot of speculation. All we know is that this planet is in the “Habitable Zone” of its star: That means, there is a good chance that liquid water can (or may) exist on the surface of this planet. That’s all we can say..for now.

And what does this mean for any “Alien” life out there? Well, to know if there is any life out there on Kepler-186f, we need to look at this planet's atmosphere and see if there are any signs of life. Why do we need to look at a planet's atmosphere to find life signatures?  We are assuming that any life on the surface interacts with it's atmosphere (just like life does here on Earth, by releasing various gases), and this may give us a hint about any life, if it exists, on Kepler-186f. But this system is too far away (about 500 light-years from us). Our telescopes are not sensitive enough to observe the planet's atmosphere.But there is a reason to be optimistic: This discovery proves that there can be Earth-size planets in the "Habitable Zones" of other stars ! Consider this: you are looking for a nice house in a pleasant neighborhood, with some kind of options in mind: like a nice yard or large rooms or multiple-levels. Just because you have these in mind doesn't mean that kind of a house exists. But if you find one, then you know there may be more.

The most interesting part is just starting: Stars like Kepler-186, the cool and dim ones which do not emit much light, are the most common stars in our  Galaxy. We now know there are a lot of Earth-size planets in the “Habitable Zones” of these cool stars.  In fact, Kepler mission  discovered nearly 3000 planet “candidates”  (that means we are not yet sure all these 3000 are planets, we have to confirm that they are actually planets). And we have already found many planets in and around the “Habitable Zone”, but all of them are larger and probably more massive than Kepler-186f.

Habitable Zones around different kinds of stars. Hot stars are at the top, cool stars are at the bottom. The “flux” here means the amount of starlight the planet will receive with respect to the Sunlight on Earth.  So a “1” indicates that a planet gets the same light from its star as the Earth from the Sun. A 0.75 means the planet receives only 75% of light, and so on. Some of the currently known exoplanets in the” Habitable Zone” (blue shaded region) are also shown. Can you find Kepler-186f?  Credit: Chester Harman.

Now, let’s ask the question again: If there are so many planets out there, and that too within the “Habitable Zone” of their stars, why don’t we see any “Aliens”? Where are they? Imagine that you moved into your new house in a new neighborhood. You look around to introduce yourself to your neighbors. You see lots of houses around you. But you don’t see or hear any people….not beside your house, or even on your street…or in your area..…just houses. Wouldn’t that spook you?