Saturday, October 11, 2014

How Common Are Potential Habitable Worlds In Our Galaxy?

I have written a guest post for 'Centauri Dreams' website about the commonness  of potential habitable worlds in our Galaxy. It can be accessed here.

Sunday, June 8, 2014

My God! It's full of galaxies!

How many stars we can see with the naked eye on a clear, moonless night sky? Perhaps 5000 stars? May be 6000? Or may be 3000? Whatever is the number, it is in "thousands". It is also possible to see some foggy star clusters that are surrounding our 'Milkyway' Galaxy. Come to think of it, we can see our own Milkyway galaxy if you have a very clear night! (I highly recommend once in your lifetime to go out, and watch the Milkyway. It is awe inspiring!)  And you can also see our nearest neighboring galaxy (Andromeda).

What if we want to see galaxies much further away? And how far 'away' can we see? How *many* galaxies can we see? Fortunately, we have NASA's Hubble Space Telescope (HST) that can answer some of these questions. Astronomers asked HST to peer at one location in the sky, for a period of time, and created a picture of that sliver of the Universe. Here is that picture:
Fig. 1: An image of the Universe, from ultraviolet to infra-red part of the light, in one tiny part of the night sky, created by Hubble Space Telescope. There are about 10,000 galaxies in this picture. Some are so far away that the Universe was just a few hundred million years old when they were formed. Image courtesy: Hubble site
To say this picture is astonishing doesn't do justice to what we are seeing.  Almost every single point of light you see in this picture is a galaxy! Some are bigger, some are smaller than our own Milkyway galaxy. Since all of these galaxies are millions of light-years  away from us, that means we are seeing them how they were millions of years ago (because it took light that much time to reach us).  So there is also a "depth" (of time) included in this picture. Basically, we are looking back in time in the history of the Universe. It's like looking at your pictures, from childhood to the present, keeping the oldest photos further from us.

How big is this picture? It is 2.4 arc minutes wide. Well, that doesn't help. So what does it mean? Here is something that I thought would help. If you extend your arm and place your little finger up in the sky (see figure below), then the width of the little finger is about 1 degree.
Fig.2 : If I extend my arm out, pointing little finger to sky, the width of my finger makes 1 degree width in the sky.

Now, let me make a square box of 1 degree on each side.
Fig.3: A square box of 1 degree in the sky, compared to my little finger.  

There! Now, the size of the Hubble image shown in Fig. 1 would appear like this:
Fig. 4: The size of the Hubble image on the sky, compared to my little finger.
Can't see it? Here is a zoomed in picture:

Fig. 5: Zoomed in view of Fig. 4 above. The dot (or the tiny yellow square) on the left is the size of the Hubble image. The dot (or the black square) on the right is the actual Hubble image shown in Fig. 1 above.
 All those 10,000 galaxies that you see in Fig. 1 are in that tiny black dot above the tip of my little finger! And that is just a tiny speck of the sky.  Now, Imagine if you can do this on the whole sky (there would be nearly 25 million of those tiny black squares to fill up the sky! Note: this is a bit of an overestimate as I assumed the box is a 2.4 arc minute square). THAT is the observable Universe!

Next time when you look up in the sky, extend your little finger up and recall the above picture.  And realize that the Universe is always there, reminding us the insignificance of our momentary existence on a third planet going around a yellow star in one corner of a galaxy among billion others.

Monday, June 2, 2014

Kepler-10c: Our planet's super-hot, overweight cousin

A team of astronomers announced today that they have discovered a planet (or rather, revised the mass estimate of a previously discovered planet) that is 17 times massive than our Earth, and 2.35 times as large. The planet is orbiting around a star called "Kepler-10" every 45 days and is designated as Kepler-10c, the second planet in that system. Since the star is very similar to our Sun (but a lot older ~ 10 to 11 billion years, compared to our Sun's 4.5 billion years), the short orbital period means that the planet is very hot (~310 C, about three times the boiling point of water).  The first planet in this system, Kepler-10b, which is about three times more massive than the Earth, is even worse: It orbits the star every 0.83 days (20 hours)!, making it one of the hottest rocky planet  (~1895 C, about the melting point of the metal Chromium).

These two planets were discovered back in 2011. At that time, astronomers could only put an upper limit on the mass estimate of Kepler-10c, ~ 20 Earth mass, the second planet in the system. Now, they have obtained more data and found a more precise estimate of the mass: 17.2 Earth mass. This is surprising, considering that the radius of the planet is 'only' 2.35 Earth-radii. And that means it has a higher density than the Earth (Kepler-10c density = 7.7 grams/cm^3, Earth density = 5.5 grams/cm^3), which in turn means it has a rocky composition.
 Fig.1: Artist's impression of the Kepler system. Kepler-10b (red) is the closest planet, and Kepler-10c is the farthest planet. The star itself is very similar to the Sun, but much older (~ 10 billion years). Image credit: Harvard press release

Now, just giving you the numbers about mass, radius of this planet doesn't make quite an impression, or the convey the significance, of this discovery. So here is a cartoon drawing that I thought would help (I hope!).
Fig. 2: Comparing the size of Kepler-10c with Earth and other smaller extrasolar planets. Kepler-10c is one of the densest terrestrial planet discovered to date.

Assuming a typical 'stick-figure' person's height represents the radius of a planet, and the size of the belly represents the mass, Fig. 2 shows comparisons of different extrasolar planets, including Kepler-10c, to Earth. Look how squished is our Kepler-10c cousin! And for a good reason: Our cousin is trying to tuck his body mass into a similar height as some of the smaller planets. The only way he could do it is to squish or compress enough to fit inside that height! When you try to do this, the amount of mass per 'height' increases. In other words, our cousin is trying to put more of his body mass in per unit area than us. So his density increases; which is  the case for Kepler-10c.

Our traditional/general view is that planets massive than 10 to 12 Earth masses are probably not rocky, because beyond this limit they accumulate gas from surroundings during their formation, and become gas giants (like Jupiter or Saturn) or ice giants (like Neptune or Uranus). But Kepler-10c, with a mass of 17 Earth masses, which is way beyond the 10 Earth mass limit, appears to be rocky. There could be many reasons (may be it formed after gas dissipated in that system, accumulating only solid particles. Or may be it indeed accumulated gas, but being so close to it's star, the gas envelope got blown away, and what we see is only the left over mantle/core of the once gas-planet. The star is old, so the planet had some time to bask in this radiation). We don't know....yet.

But this discovery could pave the way for more such planet discoveries, probably in the habitable zones. And it could also mean that potential habitable worlds come in all sizes and masses, each with unique characteristics.

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?