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The Earth-Sized World Next Door

 “listen; there’s a hell of a good universe next door: let’s go.”  – e e cummings

News Flash:  Astronomers have discovered a rocky planet that is a virtual twin in size to Earth, in orbit around a sun-like star!

This is the headline we hope for sometime in the next few years when NASA’s Kepler satellite, launched in March, has had time to work its magic.  Kepler is keenly observing the shining of more than 100,000 stars, watching for the slight telltale dimming caused by transiting planets.

Actually, the above news is actually centuries old.  We’ve known of such a planet since the 18^th century when transit observations allowed us to deduce that, shockingly, Venus – the planet next door – is very nearly the same size as Earth.  As soon as we knew the distance to the Sun – obtained through careful observations of transits of Venus – we could use the apparent size of Venus to calculate an exact diameter of the planet.

The nearly identical size of Venus, along with its close proximity, and the obscuration of its surface by globally enveloping clouds, contributed to the enduring pre-space age mythology of Venus as another Earth – a reverie that was rudely interrupted by planetary exploration.  Venus turned out to have surface conditions shockingly hot and dry, forbidding to life and utterly alien to our Earthly expectations.

Yet planetary exploration has also taught us the importance of size in determining the evolution of a planet’s interior, surface, atmosphere and – most likely – life.  The emphasis on finding other planets the same size as Earth comes from the realization that size has controlled our planet’s interior thermal evolution and prevented the escape of its atmosphere.  These factors have allowed Earth’s global system of plate tectonics and the deep convection patterns that maintain it, along with its self-regulating, life-enabling climate system.  We know that much of what gives Earth its character is related to its size, but we don’t yet know how sensitive to size these planetary processes are.  That is one of the big questions that the new age of comparative planetology in the post-Kepler era will allow us to answer.  Understanding the evolution of Earth-sized planets is absolutely essential for contextualizing our own existence and the likelihood that we have company out there among the worlds.

Kepler will hopefully tell us, in a statistical sense, how many Earth-sized worlds are out there.  But we will not be able to explore any of these worlds, up close, within our lifetimes or even within the foreseeable future.  Meanwhile, Venus sits there nearby but mostly unexplored.  In recent years Venus has been largely neglected by spacecraft explorers from Earth.  This trend may be coming to a close with the success of the European Space Agency’s Venus Express orbiter and the planned launch next year of the Japanese Venus Climate Orbiter.

However, some of the biggest and most important mysteries of Venus will be hard to crack with orbiters.  Were there once oceans there?  If so, what happened?  If not, why not? How, when and why did the climate change to its current extreme state?  Did Venus ever have life, and what happened to it? Are the volcanoes we see in radar images currently active?  How could an Earth-sized planet, so nearby, evolve into such an extremely alien environment?  What are the implications for the distribution of habitable planets in the universe?

To crack these mysteries we will need to brave the challenging surface and atmosphere of Venus with landers, entry probes and balloons.  NASA, the European Space Agency and Russia are all seriously considering new plans for more ambitious missions.  In 2008 NASA commissioned a Science and Technology Definition Team to draw up detailed plans for a possible multi-billion dollar Venus “flagship” mission which could launch in 2020.  Other smaller, innovative missions could launch well before this.

Venus is not low-hanging fruit: It’s a tough place to explore.  But now, as we extend our view outwards into the galaxy, seeking to know the variety of Earthlike worlds, its also time to venture out and look more closely at the Earth-sized world next door.