The family of confirmed planets that exist outside our solar system gained fifty new members last week when the High Accuracy Radial velocity Planet Searcher (HARPS) team announced new discoveries at the Extreme Solar System Conference in Wyoming. This announcement described a planet that may orbit around its star in a habitable zone, known as a “Goldilocks zone” because it is neither too hot nor too cold, allowing liquid water to form. Liquid water is a necessary precondition for life as we know it, so finding planets that may have it is an obvious first step to finding evidence of extraterrestrial life. This brings the total number of known exoplanets – planets that exist outside our solar system – up to 685, with thousands more potential worlds waiting to be confirmed by various scientific missions.
This plethora of exoplanetary discoveries has seriously challenged scientists’ ideas about planetary formation. The traditional model has planets forming around the same time as a newly born star, accreting from the same disc of dust that rotates around the protostar, an early phase in the formation of a star. But with new discoveries, it became clear that current models were unable to predict the diversity of planets in the galaxy. “These models are crap. They may be the best we can do, but they are still crap,” said Hal Levinson, an astronomer speaking at the annual American Astronomical Society meeting in January.
Scientists use a variety of techniques to actually detect these planets, ranging from directly imaging them with powerful telescopes to more indirect methods. One such method detects the subtle periodic wobble of a star and its planet as they rotate about each other in their cosmic dance. This tiny effect can be measured from hundreds of light years away. It also allows astronomers to figure out the mass and orbital period of the planets. This is possible even in solar systems with multiple planets. The HARPS team has found dozens of planets, including a considerable number of “super earths” (planets with masses of up to 10 times that of earth). However, the radial velocity technique doesn’t reveal anything about the density of the planet, which means the “super earths” could range from puffy gas balls, such as those found in our own outer solar system, to more compact, rocky planets like earth.
The radial technique for detecting planets favours finding very large celestial bodies closely orbiting their suns. Many of the early exoplanet discoveries using this method were of these “hot Jupiter” planets, which orbit their stars far closer than our own Mercury, with masses considerably larger than Jupiter, at about 318 earth masses. These enormous planets would have surface temperatures reaching upwards of a few thousand degrees, whose ‘year’ could be as short as a few hours because of their tiny orbits. One planet the HARPS team found gets so hot that it actually acts like a Jupiter-sized comet, leaving a superheated ‘tail’ of planetary debris that gets blown away by the stellar wind at more than 35,000 kilometers per hour.
The transiting technique is a more direct way of measuring planet size. This technique watches stars for periodic “eclipses” caused by the planets moving in front of the star. The tiny drop in the intensity of the star’s light can be detected, and from that the physical size of the planet can be deduced. Of course, this only works when our telescopes are on the same orbital plane as the extrasolar system. The laws of probability tell us that this lucky alignment would only occur about 0.5 to 10 per cent of the time, depending on the planets distance from the star. In order to appreciate the miniscule change that occurs, imagine being able to see a poker chip placed in front of the search light in downtown Montreal all the way from Toronto. The transiting technique is not only able to do this, it does it on the scale of trillions of kilometres.
The newly operational Kepler spacecraft uses this method to find planets, watching about 145,000 stars constantly to detect these small drops in brightness. Even though it has only been in operation since 2009, it has already detected over 1200 candidate planets, including dozens of “super earths” and a couple within the habitable zone of their stars. Kepler has even detected a planet that orbits around two stars at once. Like the fictional Star Wars planet Tatooine, no two sunsets on this planet would ever be the same, because the two stars would be orbiting around each other as the planet rotates around the pair!
The transiting technique also offers scientists an opportunity to directly measure the atmosphere and temperature of the exoplanet, because the spectral signature of the starlight changes when there is a planet obstructing it. With ever-increasing telescopic power comes an increasing chance that we will finally detect a sister planet with a similar size, temperature, and atmospheric composition as Earth. This is one of the explicit missions of the upcoming James Webb Space Telescope (JWST), which is the successor to the immensely successful Hubble Space Telescope. The JWST will have a resolution of over 20 times that of the Hubble, which would make it theoretically sensitive enough to directly image a planet with compoarable size to earth. Funding for the JWST was recently called into question, citing cost overruns and mismanagement, but the project was ultimately allowed to continue. However, the earliest possible launch date for the JWST isn’t until 2019.
With the continuation and progression of all these various detection methods we truly are on the cusp of detecting a planet orbiting another sun that would be capable of supporting life as we know it. With the next generation of telescopes, we will be able to probe the atmospheres of some of these potential Earths. In doing so, we may even find a telltale chemical signature that would be incongruous with our model of a lifeless planet. Considering that scientists now know that planets can be found on up to 40 per cent of sun-like stars, and that there are billions of these stars in our galactic neighbourhood, it is virtually certain that there is extraterrestrial life in our cosmic backyard. And it will almost certainly be weirder and maybe even more wondrous than we can possibly imagine.