Nikku Madhusudhan, on life on other planets

Are we alone in the universe? Two papers published within the last year by Nikku Madhusudhan, a postdoctoral researcher in the Department of Astrophysical Sciences, offer evidence suggesting that the presence of water on other planets – a precondition for life as we know it – may be less common than scientists had suspected. Madhusudhan studied a Jupiter-sized planet, known as WASP-12b, that is orbiting a star in another part of our galaxy. He concluded that the planet’s atmosphere contains more carbon than oxygen, contrary to what we find in our solar system, and that it probably contains less than 1 percent of the water scientists expected. He discussed his findings with PAW.

WASP-12b is about 1,200 light-years away from Earth. How can we tell what it is made of?
By observing light emitted from the planet at different wavelengths, we can infer its atmospheric composition. From that we can make deductions about what it could be made of and how it could have formed. Ever since astronomers discovered the first planets orbiting other stars about 15 years ago, we generally have been assuming that other solar systems have similar chemical compositions to ours. In our solar system, there is twice as much oxygen per hydrogen atom as there is carbon, and we have been assuming that is true everywhere. That hypothesis leads to a conclusion that, given the right temperatures, there can be a lot of water on other planets, as there is on Earth.

But what if the carbon-to-oxygen ratio in other solar systems is not like it is in ours? What if carbon is more abundant? We made many observations using NASA’s Spitzer space telescope, and last year, for the first time, we had enough data to measure the carbon-to-oxygen ratio in the atmosphere of a planet outside our solar system. 

What did you find?
The star that WASP-12b revolves around has a carbon-to-oxygen ratio similar to our sun. But to our surprise, we found that the atmosphere of WASP-12b itself contains as much or more carbon than oxygen. We call that a carbon-rich environment, as opposed to an oxygen-rich environment. It is completely different from what we know in our solar system. This is a game-changer, because if that atmospheric concentration is indicative of the planet’s interior, it suggests that the planet was formed in conditions unlike what we know of how planets in our solar system were formed. 

How were those planets formed? 
We hypothesize that, in order to make a carbon-rich planet around an oxygen-rich star, there must have been a way to deplete that oxygen when the planet was formed. We don’t know what that process was yet. But we have developed a model that suggests that, if you took out the required amount of oxygen, water ice practically disappears from the planet’s formation region. This has serious implications in many ways. For example, if rocky planets formed in such conditions, they could have very little water, making them much less likely to host life as we know it. 

What does that suggest about life in other parts of the universe?
All the planets we can observe at this level of detail are in our own galaxy, so we are not trying to suggest anything about other galaxies. We need to obtain more data to validate our present results first. So far, our findings have not been contradicted. It means that we can’t just assume that planets elsewhere will be filled with water, even if the temperatures were right. 

Time magazine suggested that some of these carbon-rich planets could be made entirely of diamonds. Is that possible?
It’s partly sci-fi, but there is also an element of truth to it. If it is true that a gas-giant planet like WASP-12b has a carbon-rich atmosphere, it is easy to imagine rocky planets that are also carbon-rich. Oxygen-rich rocky planets like Earth contain a lot of silicates — sand, basically — but on a carbon-rich planet, those silicates would be replaced by many types of carbides. Such a planet could be made of diamond, or graphite. The carbon also could be in molten form. We don’t know, and it is too far away for us to ever send a probe there to find out. But we believe that a carbon-rich rocky planet could easily form a diamond mantle if the pressures were great enough.