by Prasad Ganti

In the last decade or so, there has been lot of excitement around the discovery of exoplanets, that is the planets outside of our solar system. An interesting book on this topic has been “Exoplanets” by Michael Summers and James Trefil. The pace of discovery of exoplanets has quickened since the launch of Kepler space telescope a few years back. More discoveries can be expected in the future as more such telescopes are lined up to be launched. The interest in exoplanets is to find out if there is any life outside of our Earth. The search for life continues even within our solar system. But the curiosity about how the other worlds look like and how life on them could be, persists.

The first exoplanet was discovered to be orbiting a Pulsar in the 1990s. The first normal exoplanet was found around the star 51 Pegasi. It is as big as Jupiter and is very close to its star. Closer than our Mercury is to Sun. Both of these are very strange cases. A pulsar is a dead remnant of a massive star. It is basically a neutron star which is very compact and dense, sending out a beam of radiation. A planet orbiting such a dead star? Or a massive planet orbiting so close to a regular star?

A variety of planets have been discovered. More flavors than what can be found in our solar system. Some planets are rocky and several times the size of Earth. Some planets are so light that we cannot figure out why they don’t collapse under their own gravity. Some planets made of pure carbon, with diamond mantles and cores of liquid diamond. Diamond does not exist in liquid form on Earth. Some planets are known to orbit up to four stars at a time. Some planets are so close to their stars that the rocks on their surfaces are vaporized and then fall back as rain. Then there are planets which are wandering around the galaxy, without orbiting any star. Nature never ceases to amaze us. The last case was very intriguing to me as I always imagined planets orbiting their stars.

In any solar system, a huge mass of swirling dust and gas creates the planets. The planets begin a complex dance driven by the force of gravity – what we call as a game of cosmic billiards. Planetary orbits shift around and planets can be ejected from the solar system. It is likely that there are many more rogue planets randomly moving in the galaxy than there are planets circling stars.

Planets reflect the light from their stars. But that light is too feeble to detect, given that the starlight is many time brighter and tends to mask the reflected light from the planet. But the planets do pull on the star as much as the star pulls on the planets. This pull causes a wobble of the star, which can be detected. Called the Doppler Method, it is useful for detecting larger planets closer to their stars. Also, as planets pass in front of their stars, there is dimming of light from the star. Something similar to the eclipses of the Sun and the Moon we see on the Earth. But the dimming can also occur due to sunspots, or in a double star system, when one star goes behind the other.

A planet can get heated from its interior due to radioactive decay of heavy elements like Uranium and Radium, like our Earth. Or due to tidal forces as well. Such infrared radiation (heat) can be detected, but our detectors are not as sensitive yet. Or the starlight can be filtered out to get the light from the planet alone. The James Webb Space Telescope which is scheduled to be launched in a couple of years, will have a coronagraph to do this filtering. We can determine the composition of the exoplanet’s atmosphere by directly observing the spectrum of the light. The spectrograph breaks the light into its constituent colors. Just like a prism breaks the sunlight into different colors. The spectrograph has some dark lines depending on what the atmosphere of the planet is like. The light absorbed by the elements in its atmosphere lead to these dark lines.

Kepler watches only a small patch of the sky. Despite its handicap of malfunctioning reaction wheels, it still works reasonably well. Reaction wheels are gyroscopes which keep the satellite steady. Once it identifies a candidate planet, it turns over the validation process to Hubble or other ground based telescopes. NASA will launch TESS (Transiting Exoplanet Survey Satellite) will be launched in 2018. Unlike Kepler, it will sweep the entire sky and focus on nearby stars.

The future appears to be very bright in terms of detecting the exoplanets, and possibly life outside of our Earth. We may not be alone after all!