Recent Discovery of an Earth-sized Exoplanet in Context

Last week a new Earth-sized planet was discovered in a neighbouring star system, Alpha Centauri B, which is only 4.3 light years away (Dumusque et al, 2012).  Although this planet is not likely to be a candidate for life, this discovery marks a potential threshold moment in the hunt for a habitable earth-like planet.

Scientists have officially known about the existence of exoplanets – planets beyond our Solar System – since 1992 (even though the first was technically discovered in 1988) (Campbell et al. 1988).  Before this period philosophers and scientists hypothesized that exoplanets existed but there was no empirical evidence to support that assertion.  During the early years of exoplanet detection astronomers had no idea how other solar system would be organized or what type of exoplanets would be found.  From the very beginning the first exoplanets challenged planet and solar system formation theory because they were nothing like we had observed before.  The first exoplanet was found around a pulsar (supernova remnant) and is now thought to have formed in a ‘second round’ of planet formation after the death of the original solar system post-supernova.  Most planets found after that throughout the 1990s were what astronomers called ‘hot Jupiters’.  These planets were large gas giants (like Jupiter) but were found very close to their parent star (around the same distance as Mercury is to our star).  These worlds were bizarre with orbits, sizes, formation histories and climates that were alien compared to the planets and structure of our Solar System.  These early discoveries also revealed that many solar systems could not support the stable existence of an earth-like planet.

However, throughout the past ten years exoplanet detecting technologies improved and exoplanet hunting became an exciting field of research.  The pace of exoplanet finds has been accelerating at a near-exponential pace (Schneider, 2011) and it seems like every year astronomers get closer and closer to finding that ‘goldilocks’ planet.  Some planets have been found directly by using techniques like gravitational microlensing, and some planets have been found indirectly by using techniques like radial velocity.

Throughout the early 2000s scientists started to report on the existence of ‘super-Earths’, which were terrestrial planets double or triple the size of Earth.  These planets orbits and atmospheric composition varies tremendously, there is evidence for water on a few of them, but none found to date are likely to be in the habitable zone around their host star.  Despite this, these planets represent a great opportunity for astronomers to find a potentially habitable planet and because of that the European Space Agency (ESA) will be deploying the CHaracterising ExOPlanets Satellite (Cheops).  Cheops has been designed specifically to hunt for a super-Earth within the goldilocks zone of its host star and as a result identify nearby habitable planets.

As promising as super-Earths seem to be, the holy grail of the exoplanet search is finding an Earth-sized planet in the goldilocks zone.  NASA’s Kepler telescope partially achieved this last year whenastronomers detected the first Earth-sized planet located in the Kepler-20 star system (946 light years from Earth and with 4 other planets) (Fressin et al., 2011).  However, this planet was much closer to its host star than Earth is to our sun and therefore is probably more like Venus than Earth.

The planet found last week, tentatively titled Alpha Centauri Bb, is lighter than the Earth-sized planet found in the Kepler-20 star system, and unfortunately not within the habitable zone, but it is only 4.3 light years away (as opposed to 946 light years away) (Dumusque et al., 2012).  This means that astronomers now have evidence, which indicates Earth-sized planets are just as numerous as larger planets and that they exist in our stellar neighbourhood.  Although this is not as profound or significant a finding as finding a planet like Earth, it is a significant step in the right direction, and a finding that would have been unimaginable only 20 short years ago.  And perhaps more importantly, this finding continues an impressive trend in astronomy towards more refined detection techniques that have continually revealing new planets that are ever more Earth-like.  To me, this indicates that if the rate of exoplanet discovery continues, we may find a planet just like Earth in a few short years.  If true, it would forever change our understanding and perspective of humanity and Earth.


Campbell et al., 1988.  A search for substellar companions to solar-type stars.  Astrophysical Journal, 331: 902-921.

Dumusque et al., 2012.  An Earth-mass planet orbiting a Centauri B.  Nature

Fressin et al., 2011.  Two Earth-sized planets orbiting Kepler-20.  Nature, 482: 195-198.

Schneider, J.  2011.  Interactive extra-solar planets catalog.  The Extrasolar Planets Encyclodia.


About Cadell Last
Hello. I'm probably drinking coffee and reading.

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