The Search for Other Planets Like Earth

ESO/L. Calçada

An artist’s impression of the Gliese 581 solar system, which lies a relatively close 20 light-years from Earth. The system is home to at least three planets, one of which could be potentially habitable.

Until the discovery of the first extrasolar planets in the mid-nineties, the search for other planets like Earth was something of science fiction. Today, we are discovering more and more alien worlds orbiting stars other than our own. At the time of writing, the existence of 3533 extrasolar planets had been confirmed, according to the Exoplanet Catalogue. Of the confirmed findings, the majority are gas giants like Jupiter, since they’re easier to spot. However, new discoveries are pouring in all the time, and dozens of these lie within the habitable zones of their host stars.

The search for another Earth may be young, but it is making incredible progress in minimal time. The number of exoplanets being discovered is increasing exponentially every year, and it is now widely suspected that our galaxy alone contains billions of terrestrial alien worlds. The launch of the Kepler space observatory by NASA in 2009 is entirely dedicated to the search for exoplanets, and there are many smaller programs in operation as well, both publicly and privately funded.

While it certainly still seems that life in the universe is rare, and Earth-like life is likely many times rarer, the laws of probability alone should give us some hope. Due to the sheer size of the universe and the commonness of materials required for life to evolve, life simply must be out there somewhere. It is likely just a matter of time before our search for another Earth comes to an end in what will be one of the most epic discoveries of our time.

The Search Begins

Exoplanets have been thought to exist for hundreds of years among scientists, but no proof to support these claims came to light until the 1990s. Detecting extrasolar planets is an extremely difficult challenge and, until recently, the technology simply did exist to make this possible. It was not until 1992 that the hunt for exoplanets started getting results with the discovery of a planet orbiting a pulsar star some 1,000 light-years away. In 1995, the first completely definitive detection of an extrasolar planet was confirmed, and a new era in space exploration came into being.

How It’s Done

Searching for another Earth, or any other exoplanet, is very challenging. Planets are many times smaller and dimmer than the stars they orbit. Compare, for example, the Earth to our own star. The Earth is 109 times narrower than the Sun, and our planet produces no light of its own. Because of this, exoplanets can rarely be detected directly and instead need to be discovered by the effects they have on their host stars and other surroundings.

The Kepler mission, among others, uses the transiting method to detect exoplanets, and this has proven to be one the most successful methods so far. As a planet passes in front of its host star, the amount of light given off by that star decreases by a miniscule amount. Using very carefully calibrated photometric cameras, such as those on the Kepler space telescope, it is possible to deduce important characteristics such as the planet’s size and distance from its host star. The method does report a great deal of false findings, however. Therefore, such discoveries require confirmations.

Some extrasolar planets have been directly imaged, but all of these are larger than Jupiter and emit enough infrared light reflected by their host stars to be seen by powerful enough telescopes.

What Exactly Are We Looking For?

ESO

This illustration from the European Southern Observatory shows the estimated habitable zones of main sequence stars.

Although there could well be alternative biochemistries out there and unimaginably different forms of life and ecosystems based on different materials to those required by Earth-like life, it is better to search for what we know. Earth is the perfect world for Earth-like life and it has a huge range of characteristics which are not likely to be widely replicated elsewhere in the universe. Fortunately, however, due to the sheer vastness of space, there is still plenty of room for some amazing coincidences.

The search for other planets like Earth means finding a planet which we could, in theory, inhabit with minimal or no requirement for adaptation. This means a planet that lies in the sweet-spot of a solar system where it is not too close to the host star that it is too hot or too far away where it would be too cold. An Earth-like world would have to be massive enough to have a gravitational pull not too dissimilar to Earth. This would allow it to retain an atmosphere which, in turn, would have to be composed of nitrogen and oxygen.

A whole range of other features are also important. Earth-like life needs water – the air pressure and temperature of the planet’s surface needs to be just right so that water can exist as a solid, liquid and gas. Other things that have been essential to the evolution of life as we know it include a moon with powerful enough tidal forces, a magnetic field to protect the surface from lethal cosmic radiation and a hot, spinning dynamo of a core to keep the world geologically active. Without these perfect conditions, life as we know it would never have had a chance to evolve.

There are plenty of exceptions to what generally constitutes a place that human beings or, at least, some form of Earth-like life, would find inhabitable. Just because an Earth-size planet is found in the habitable zone around a star still doesn’t mean that it has a good chance of being hospitable. In our own solar system, we have Venus, for example and, while it lies within the habitable zone, it is one of the most hostile environments in the solar system due to various internal factors, namely a runaway greenhouse effect that makes its surface hot enough to melt lead.

On the flip-side, Earth-like life could thrive on a moon rather than a planet. A gas giant planet orbiting in a star’s habitable zone certainly won’t be habitable itself but, if it has a large enough moon that also fits other important criteria, then it could also be habitable. Extrasolar moons are vastly more difficult to detect than even exoplanets, but this is not to say we won’t find any in the coming years. After all, the Kepler programme has already collected a few possible candidates.

The Search So Far

ESO/M. Kornmesser

An artist’s impression of the planet Proxima Centauri b. Located in the nearest solar system to our own, Proxima Centauri b is not likely to have a habitable surface due to extremely high radiation. The two smaller stars in the background are Alpha Centauri A and B, two sun-like stars that, combined with Proxima Centauri, make up a triple-star system.

The Planetary Habitability Laboratory currently lists 44 potentially habitable exoplanets, as of October, 2016. After only a few years of hunting for another Earth, this is a highly encouraging statistic. Of these 44, only 10 lie within the conservative habitable zone, while the rest lie within a wider and more optimistic set of parameters. However, many of these candidates, such as Proxima Centauri b, Gliese 581 c and Gliese 667 Cc, orbit red dwarf stars which, while being by far the most plentiful in the universe, are very different to our own. Most notably, any planet that lies within the supposed habitable zone of a red dwarf is likely to be tidally locked to its host, meaning that one side of the planet always faces the sun. While this does not necessarily preclude the possibility of a niche for life around the twilight zone of the planet, there’s little chance that any such a world will be remotely like our own.

Currently, one of the most promising candidates for another Earth is Kepler-62e, a planet orbiting a star similar to our own and around 60% larger than Earth. Interestingly, however, computer simulations have shown that planets of this size are likely to be complete covered by an extremely deep ocean. Another is Kepler-452b, a planet around 1.5 times the diameter of Earth and orbiting a G-class star like our own. Unfortunately, however, at a distance of 1,400 light-years, it would take 26 million years to get there using current technology.

Conclusion

As methods of exoplanet detection continue to improve, scientists are learning more and more about the numerous other worlds out there. Methods to detect essential characteristics about surface composition and atmospheric conditions are getting more sophisticated, allowing us to narrow down the search. The search is still very young but, with new discoveries being made every month, it seems highly likely that our search for a new Earth will soon yield some very exciting results. If and when (and it’s probably more a matter of when) that happens, the time will come when we have to take the next step – to find a way to get there, across the unimaginably vast voids of space.