Discovering the New Beginning

The following post was written by Professor Stephen Kane, who has been researching planets around other stars for almost 20 years and has discovered and characterized hundreds of exoplanets, including Kepler-186f - which is the smallest planet yet to have been found in the Habitable Zone of a star. After spending many years working at the NASA Exoplanet Science Institute, Kane is now a Professor of Astrophysics at San Francisco State University.

It is my opinion that the past two decades have brought about a significant turning point in human civilization. Allow me to explain.

The first planet discovered around a star similar to our sun occurred in the year 1995. The name of the star is 51 Pegasi located in the constellation of Pegasus. The star is about 50 light years from the Earth and is visible to the naked-eye if you manage to find dark skies away from the city lights. The planet which orbits the star was unlike anything that was previously known; a giant planet much like Jupiter but in an orbit that only takes 4 days to complete. It turned our ideas of what a solar system should look like upside-down. My name is Stephen Kane and I began researching planets around other stars (“exoplanets”) in the beginning of 1996 and in the wake of the confusing discovery that had occurred. Almost 20 years and hundreds of discoveries later, the universe has shown that it is full of surprises and the diversity of exoplanetary systems is quite simply astounding.

One thing that is certainly true is that we are better at discovering planets now than we were almost 20 years ago, both in terms of the discovery rate and our ability to detect smaller planets. The graph above demonstrates this and shows that with each passing year we are pushing deeper and deeper into the unknown. In the first few years we were discovering the giant Jupiter-like planets because those are the easiest to detect due to their large size and mass. In the last couple of years though we have crossed a significant threshold that will always be remembered: we as a civilization now have the ability to remotely sense the presence of other Earth-size planets!

A remarkable example of this is the data point shown just above the green “Earth line” which represents a planet discovery made in 2012. The planet orbits the nearest star to the solar system that is visible to the naked eye: Alpha Centauri. The star is actually a binary system, consisting of two primary components; Alpha Centauri A and B, two stars orbiting each other every 80 years. This star system has captured the imagination of science fiction writers for many years, most notably as the auspicious finishing line of Sid Meier's Civilization series of computer games. The reason for this is clear: it represents the ability of humankind to overcome and rise above the struggle to survive on Earth and achieve our destiny of exploring the universe. What we did not know until 2 years ago is that Alpha Centauri B does indeed harbor an Earth-mass planet, albeit very close to the star and thus very hot, but still an indicator that other planets likely reside in the near vicinity.

The “Holy Grail” of planet hunting however is the discovery of Earth-size planets which are just the right distance from their star such that habitable conditions might exist at the surface. When one thinks about the conditions which favor a habitable planetary surface we may think about the size of the planet, the kind of atmosphere it might have, and how far the planet is from the host star. All these factors enable one primary habitable feature: the presence of liquid water. In our own solar system we have a striking example in our sister-planet Venus of where surface conditions can go spectacularly wrong and become as uninhabitable as one could imagine. Venus is too close to the Sun to have been able to retain its liquid water and now has a “runaway greenhouse” atmosphere consisting mostly of carbon dioxide. Can we use this information to predict where other habitable planets might lie?

The answer is a resounding “yes”. With modern knowledge of the Earth's atmosphere and the ability to create detailed computer-generated climate models, we have been able to determine the amount of solar energy needed for Earth to have liquid water at the surface. Applying these calculations to other stars, we can define a region around them called the “Habitable Zone”. Thus, finding planets in the Habitable Zone has been a major undertaking in recent years as our discovery of Earth-size planets coincides with our detailed calculations of climate models.

Earlier this year, myself and a group of other astronomers from around the globe analyzed data from a NASA telescope called Kepler. The data from this telescope revealed one of the most significant findings in the history of exoplanetary science and the achievement of another milestone in human history. We had discovered an Earth-size planet in the Habitable Zone of its star. The planet has the rather unromantic title of Kepler-186f, but in all other respects it is a discovery which inspires the imagination. The planet is only slightly larger than the Earth and orbits at just the right distance to be an excellent candidate for having a habitable environment at the surface including oceans of liquid water. The graphic below shows an artist's impression of what the planet may look like.

The exciting aspect of crossing this scientific threshold is that the prospect of Earth-like planets orbiting other stars has previously been confined to the imagination of science fiction writings. We have now entered an era where the line between science fiction and science fact has been shattered and we can now talk about other planetary systems in real-life terms. The data proving their existence is a tangible commodity that we can observe and understand. We no longer need to think of Earth as a single small oasis in a vast universe because we know that potential other homes lie in abundance and that if we can overcome the challenges of interstellar travel then we can realize both the closing scenes of previous Civilization games and the opening scene of Civilization: Beyond Earth.

We have come very far in our exoplanet discoveries in a relatively short period. There is much data to be collected and understood which will keep us busy for years to come. It is clear though that we are only seeing the tip of the iceberg in terms of the diversity of planets and the years ahead will undoubtedly reveal many more potential locations for humankind to establish a New Beginning.