In a collaborative effort between members of five institutions, scientists have discovered the most promising Earth-like exoplanet to date. The nearly Earth-sized planet, projected to live within the habitable zone of its dim, red parent star, and to be composed of 75% ice or liquid water enveloped in a substantial atmosphere, may be the first known water world in existence. GJ 1214b, as it is called, is the product of an on-going survey project poised to yield further groundbreaking results in the search for life.
A physical reality was born out of the stuff of imagination when a little less than fifteen years ago the first planet was discovered outside of our solar system—an “exoplanet”, so it was termed, short for “extrasolar planet”, suspended in deep space and gravitationally bound to a faraway star. It was the spark that touched off a modern explosion of progress in an age old quest. The same impetus which had once set us afloat the unknown open seas has now trained our eyes on the sky. We’re looking for movement. A tiny flicker of a star which might give away the existence of another world like ours.
The biggest and closest planets were the first to be discovered. This is natural given our current methods of detection, largely indirect, which depend on recognizing gravitational or visual aberrations due to the presence of these dark bodies. “Hot Jupiters”, they were called– huge, gaseous planets which gravitate so near to their host stars that a complete orbit is made every few days. A fascinating find, but with surface temperatures of a few thousand degrees Celsius, these planets are unsupportive of life as we understand it to be.
As we tuned our instruments, painstakingly improving their precision, more planets came into view. This next generation of exoplanets were termed “Super Earths”. They were smaller, denser, with radii and masses only exceeding those of Earth by several fold. Their surfaces were sometimes solid or fluid. They inched ever closer to the all-important habitable zone, the narrow region around a star where water, the solvent for all carbon-based lifeforms, can exist in its liquid form.
GJ 1214b belongs in this category of exoplanets. Slightly larger than Earth, in the constellation Ophiuchus, it emerges as the most promising candidate yet in the ongoing search for life.
Earlier, using similar detection and analysis techniques, two such Super Earths had made headlines. Gliese 581D, which orbits a dim, red star and Corot-7b which belongs to a yellow star much like our Sun. The first is estimated to be gaseous, and to reside, despite its close proximity, inside a habitable zone, due to the low temperature of slow-burning red dwarf stars. The latter is the smallest exoplanet ever discovered, with a radius only 1.7 times that of Earth and a density which hints at a mostly iron composition. But the search for life on these two planets is stinted by several factors. Gliese 581D’s plane of orbit is inclined relative to our line of sight, which not only hinders precise determinations of its mass and size but also precludes direct or indirect study of its composition and atmosphere. On the other hand, Corot-7b, due to its close proximity to its bright sun, is conjectured to be essentially a big ball of lava.
The new kid on the block, the planet GJ 1214b, with a mass 6.5 times the mass of Earth and a radius 2.7 times Earth’s radius, belongs to another dim, red star. Current data predicts an average density comparable to that of water, a little lighter, possibly indicative of a substantial gaseous atmosphere. On top of that, the planet, though tightly bound to its sun with an orbital period of only a day and a half, is potentially habitable. It is not inconceivable that on this planet under a huge red sun, some or all of that water may be in its liquid form.
GJ 1214b was discovered by the transit method of exoplanet detection. This is made possible only by GJ 1214b’s special orbit, which takes it periodically in front of its parent star as seen from the Earth. Each time it crosses our line of sight to the star, the small planet occults a portion of the star’s light. This is perceived by the instruments as a slight dip in the star’s intensity. Measured over several orbits, these flutters, as shallow as 1% in depth, allude to the presence of a dark orbiting body. Follow up measurements that detect very slight wobbles in the position of the parent star due to the gravitational effects of the orbiting planet then confirm its existence. The magnitude of the first effect is proportional to the size of the planet, for the second, to its mass. Given the parent star’s estimated size and mass, the planet’s measurements can be computed to appropriate uncertainties.
But that’s not all. The light that reaches our telescopes, having reflected off or passed through the gaseous envelope of GJ 1214b, can be analyzed to reveal the content of its atmosphere. Each element, when present on the planet, is responsible for emitting and absorbing a signature set of wavelengths of light. Is the air thick with carbon dioxide, like Venus? Full of nitrogen, like Earth? Mostly water? Or something else altogether?
This promising research continues on. Even now, the MEarth Project is scanning the skies for other transiting planets around dim, red stars, as our most profound expectations are still waiting to be met. We push ever closer to the final generation of exoplanets: Earths.
An old piece: from 2010