The Kepler Space Telescope has found thousands of planets and planet candidates. Perhaps one of the most exciting discoveries is that smaller, rocky planets are more common than we thought. Our current understanding of what makes a planet habitable for life includes a planetary surface (something you can stand on) and the potential for liquid water. If a planet is too massive, it is likely to be a gas giant, like Jupiter, with no definable surface. But there is one type of planet for which there is no Solar System equivalent. Planets that are a bit bigger than Earth, but smaller than Uranus or Neptune could possibly have a solid surface. We call these planets Super-Earths, and Kepler recently found two very interesting ones that have been making headlines (1) across the globe: Kepler 62e & Kepler 62f (Figure 1).
Kepler 62 e & f are part of a multi-planet system with 5 planets (planets being named in order of discovery by star name + a/b/c/d/etc.), but what makes these two planets interesting is that they are both in the traditional habitable zone, the region around a star with temperatures conducive to liquid water, around the same star. The inner planet, 62 e, is most likely a steamy ocean planet, like the Earth but with no continents (Figure 2). The outer one is also likely an ocean planet, but would have to have over 1.6 bars of carbon dioxide, or CO2, in the atmosphere (over 4000 times the CO2 on our Earth) to keep above freezing. This amount of CO2 would be toxic for humans, but another life form could have evolved to survive in such an atmosphere.
When these discoveries get attention from the media, they aren’t always represented accurately, leaving scientists frustrated and the public confused. This is often due to two major assumptions. One is that “habitable” means a planet has flourishing life, complete with continents, flowers, animals, and oceans. The second common assumption is that “habitable” means we could relocate to these planets if we destroy the Earth. Both of these assumptions are fundamentally flawed.
Whenever scientists use the word habitable, this means only that liquid water could exist given ideal assumptions. In the case of Kepler 62f, that means at least 1.6 bars of CO2 to eek above freezing or up to 5 bars of CO2 to have the same temperatures as Earth. Carbon dioxide is a greenhouse gas, meaning it warms a planet by absorbing infrared radiation. On Earth we are worried that we are starting to make our planet not habitable for humans by adding too much CO2 into our atmosphere, causing a run-away warming effect. But on other planets, further from their stars, large amounts of CO2 or other greenhouse gases will turn an otherwise frozen ball of ice into a temperate planet, capable of having liquid water on its surface. Of course, life may very well evolve without liquid water, using a different liquid, but when scientists first set out in our search for life beyond Earth, they will first focus on planets that have relatively temperate climates and water. Water is used by every known life form on Earth and is liquid through wide range of temperatures, in addition to being abundant in the Universe, so it’s a reasonable criterion for scientists to use until other abundant, life-enabling liquids are discovered.
The second assumption is that the main purpose of looking for habitable planets is to find an alternative planet for humans to live on. For example, in a recent article in Business Insider (2) featuring 10 habitable planets including Kepler 62 e & f, the author says, “Our planet is being destroyed. It is only a matter of time before we will need a new rock to call home.” But let us look closer into this assumption.
Kepler 62 e & f are 1,200 light years away. One light year is a measurement of distance (not time), representing the distance light travels in one year. To put it in perspective, we are just over eight light minutes away from our Sun and around 5.5 light hours from Pluto. The farthest object we have sent from Earth, the Voyager 1 space satellite launched over 35 years ago, is now just around 0.7 light days away from Earth. The nearest star to our Solar System, Alpha Centauri, is four light years away, over 70,000 times the distance to Pluto. Thus perhaps in 73,000 years Voyager 1 might be as far away as Alpha Centauri. To reach Kepler 62 e & f, it would take Voyager 21,900,000 years. The distances in astronomy are huge, almost beyond human comprehension. Granted, we will some day develop better technologies and better engines. But even still, the timescales involved in human space travel are currently a hurdle scientists don’t know how to overcome, without even considering the energy required to send a ship with humans, instead of just a tiny satellite, out into the Cosmos.
But habitable planets intrigue the imagination of the public and scientists alike because by looking at their atmospheres we might be able to finally answer the question of “Are we alone?” (3). Kepler 62 e & f are exciting because they offer two planets around the same star where we can look for signs of life someday with future telescopes. We can imagine two civilizations on each planet, with very different climates. In addition, these planets are likely water-covered with vast oceans, perhaps teeming with life. Future observations with the next generation of telescopes will help us find the answer. For now, we will have to content ourselves with the knowledge that these exotic worlds exist and continue to search the heavens for new and exciting worlds.
Sarah Rugheimer is a PhD candidate in the Astronomy Department at Harvard (www.cfa.harvard/~srugheimer). She recently submitted a paper (4) with Lisa Kaltenegger and Dimitar Sasselov modeling possible atmospheres and considering how a climate cycle might operate on an ocean world for Kepler 62 e & f.
 Two Promising Places to Live, 1,200 Light-Years From Earth by Dennis Overbye. New York Times. April 18, 2013. http://www.nytimes.com/2013/04/19/science/space/2-new-planets-are-most-earth-like-yet-scientists-say.html?pagewanted=all
 The 10 Best Replacements for Earth by Dina Spector. Business Insider. April 22, 2013. http://www.businessinsider.com/the-most-habitable-planet-2013-4?op=1
 Are We Alone? – How astronomers hope to find life in the Universe by Sarah Rugheimer. SITN Flash. January 22, 2012. http://sitn.hms.harvard.edu/flash/2012/issue110/
 L. Kaltenegger, D. Sasselov, & S. Rugheimer. Water Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features and the case of Kepler-62e and -62f. http://arxiv.org/abs/1304.5058
“This stuff is FAR” song by George Hrab