Indian search

The PRL's observatory at Mount Abu, Rajasthan.

THE seeds for the idea to establish a programme at the Physical Research Laboratory (PRL), Ahmedabad, to search for exoplanets were sown when Abhijit Chakraborty, who got a doctorate from the institute in 2000, returned there in mid-2006 after more than five years of postdoctoral research in the United States, where, he had worked with groups with expertise in searching for exoplanets. J.N. Goswami, the PRL Director, asked him whether he could initiate a dedicated programme there. “In India,” says Chakraborty, “there was no such programme, and it was felt that it was high time that someone took this up. We had some plans and we consulted some experts. It took about a few years for me to design the spectrograph [for exoplanet search].”

The PRL exoplanet search programme is based on the radial velocity (RV) measurement technique and is called the PRL Advanced Radial-velocity All-sky Search (PARAS). The instrument used to measure the RV changes is the so-called echelle spectrograph, a device that is often used in both space- and ground-based astronomy and allows astronomers to simultaneously record a wide range of wavelengths with very high spectral resolution. Chakraborty’s team began building the instrument in 2007, and according to him, perfecting the design on the drawing board itself took quite a few years and was done entirely in-house. Since Indian industry did not have the expertise to build such devices, instrument parts fabricated to design specifications were procured from different sources abroad and the instrument was assembled here.

The spectrograph has wavelength coverage from 370 nanometres to 880 nm, but the main focus for RV measurements will be 400 nm to 680 nm. The crucial component of the instrument is the detector, which is so sensitive that it can detect even a few photons. The charge-coupled device-based detector, measuring 61 millimetres × 61 mm and having pixels of size 15 square micrometre, was obtained from the company in the United Kingdom that holds the patent for the technology. The detector’s quantum efficiency, which is a measure of the fraction of photons hitting the device being converted into charge carriers, is stated to be around 90 per cent in the wavelengths of interest.

The spectrograph is now mounted on the existing 1.2-m telescope of the PRL at Mt Abu in Rajasthan, and it began taking data in April 2012. Mt Abu, which, at an altitude of about 1,700 m, is the highest peak in central and western India, has about 200 cloud-free nights a year, 150 of which are good for photometric observations, and the PARAS instrument can be used optimally for 80 nights (10 nights a month for eight months). The astronomical “seeing”, or angular resolution, that can be obtained with the telescope is about 1.1-1.2 seconds of arc. The spectrograph is installed in a low vacuum with the temperature controlled to within 0.02 °C at 25 °C. This facility is proposed to be upgraded by mounting the spectrograph on the planned 2.5-m telescope scheduled to be built by 2017-18.

Main goals of PARAS The main goals of PARAS with the 1.2-m telescope are (1) to search for massive earth-like planets (up to 20 earth masses) in the habitable zone around a sample of about 1,000 main sequence stars of G, K, M type up to 6.5 magnitude brightness with 1-2 m/s precision in detecting RV changes; (2) RV detection of non-transiting planets in the 105 square degree Kepler field; and (3) follow-up confirmations of prospective Kepler candidates transiting stars up to 10th magnitude at 7-10 m/s precision and determining the direction of the planet orbit with respect to the rotation axis of the host star.

The instrument is already performing better than expected, having achieved stable science runs with an RV stability of 1.5-1.7 m/s. For example, from the spectrum of a known sun-like star (HD88133), the original ground-based detection in 2005 of an orbiting planet was confirmed to good accuracy. How does PARAS compare with other instruments like the High Accuracy Radial Velocity Planet Searcher (HARPS) at the European Southern Observatory in La Silla, Chile, or the High Resolution Echelle Spectrometer (HIRES) at the W.M, Keck Observatory in Mauna Kea, Hawaii?

“As far as RV measurements are concerned, we are on a par with them,” says Chakraborty. “Our ultimate focus is to find planets in the habitable zone. For that, besides a high-precision instrument, which we have, we also need photons. For that, you need a large collecting area. With a 1.2-m telescope, we can do bright stars, like up to 6.5 magnitude—stars that you can see with the naked eye reasonably well. On all those stars, we can get very good measurements and we get results on a par with others. If you want to go to stars fainter than that, the photon noise with the 1.2-m telescope increases. There are not enough photons there. When we go to a 2.5-m telescope, we will be able to do better on that and we can look at planets of a few earth sizes around, even magnitude 10 stars,” he adds.

Detection of earth-size planets in the habitable zone with PARAS depends on the planet’s distance from the star. “If it is earth-like distance, it is generally difficult because it is just at the level where some people are able to detect or you really need a large telescope. That is why people are talking of projects like G-CLEF [GMT Consortium Large Earth Finder]. But what we can do with PARAS in principle is to find out the statistics of planets around not sun-like but lower-mass stars around which the habitable zone is closer. For that we need 50-60 cm/s precision, and with a 2.5 m telescope, we will be able to achieve that from the present 1 m/s. Then we may be able to find something,” says Chakraborty.

R. Ramachandran