Written by Abhijit Chakraborty
Scientists of Physical Research Laboratory, Ahmedabad, recently became the first Indians to discover a planet. Orbiting a star 600 light years away, it was confirmed as an exoplanet after observations at Mount Abu Observatory. The discovery has been reported in The Astronomical Journal. Why is it important to India, and science in general?
Why track exoplanets?
We need to understand how planets form around stars, to know if our Solar System is unique or there are many such systems, and whether the planet is at the right distance from the star so that it is neither too hot or cold to support life. We have been observing exoplanets since 2012.
What is the planet India has identified?
We were observing it (EPIC 211945201b or K2-236b) for 1½ years; this January, we realised it is indeed a planet around the star (EPIC 211945201 or K2-236). About 70% is made of iron, ice or silicates and 30% is gas. It is about 27 Earth-masses and six Earth-radii, close to Neptune in terms of mass and radius. One year on this planet is about 19.5 Earth-days and surface temperatures are 600°C, so it is uninhabitable. It is seven times nearer to its star compared to the Sun-Earth distance.
Why is the discovery important?
To detect an exoplanet is very difficult. It is like trying to spot a firefly against the bright light of a lighthouse. Direct imaging of exoplanets is almost impossible, although new techniques are being evolved by NASA and others. There are only 5-6 spectrographs around the world that can measure the mass of exoplanets at high precision (radial velocity less than 2 m/s). Apart from the US and Europe, we are the only people to have such a spectrograph.
What is the PRL spectrograph like?
We at PRL designed the PARAS (PRL Advance Radial-velocity Abu-sky Search) spectrograph and got the components custom-made, not only from India but from different parts of the world. These were brought together at the Mount Abu Observatory. Once the spectograph was built, we put in a vacuum chamber in a very stabilised environment that helps us get precise mass measurements. Starlight from the telescope is brought to the spectrograph. The spectrograph was commissioned in 2012; the search for exoplanets is an ongoing programme.
How were the experiments conducted?
The first thing is to understand an exoplanet’s characteristics and fundamental parameters like mass, radius, what kind of atmosphere it has. If you know the mass and radius, it is easy to get the density; if you have the density, you have a rough idea about the composition.
If a planet is going around a star, the star will wobble because of the presence of the planet. When we observe the star, we try to find if it is wobbling or not. Using this information, we get all the information about mass, orbit. This wobble is measured using a precise spectrograph. The radius of the planet is measured when it passes between its star and Earth. If there is a planet in between, the light of the star dims minutely. By measuring the depth of the dip in the star’s flux, we can estimate the radius of the planet. But this does not give the planet’s mass. Therefore, when NASA’s Kepler or K2 identify candidates that could be exoplanets, Doppler spectroscopy using spectrographs like PARAS are necessary to determine the wobble, and hence the mass measurement. A team of five-six used PARAS-1.
Where does planetary science in India go from here?
Our ultimate aim is to detect planets of close-to-Earth mass (2 to 10 Earth masses). ISRO is taking a lot of interest, so we are going to have a new 2.5-m telescope at Mount Abu with a bigger spectrograph. We are calling it PARAS-2. It can measure even smaller exoplanets that are two or four times Earth’s mass. This should get commissioned by 2020. Plus, we are hoping ISRO will launch some space missions relating to exoplanets.
Professor Abhijit Chakraborty is chairman of the Astronomy & Astrophysics Division at PRL, Ahmedabad. He led the research that discovered the exoplanet.
(As told to Avinash Nair)