Be Skeptical of the Hype in the Search for ‘Earth-like’ Planets

When it comes to exoplanets, "Earth-size" does not mean "Earth-like"

By Guillermo Gonzalez Published on January 29, 2015

If your only sources of information on new exoplanet discoveries are news articles like Seth Borenstein’s breathless AP report from November 2013, “Study: 8.8 billion Earth- size, just-right planets,” you will be misled about what it takes for a planet to be habitable. NASA’s Kepler space telescope has yielded a treasure trove of exoplanet discoveries — just over 4,000 at most recent count. As an astrobiologist, I am very excited to have a steady stream of new exoplanet data available to conduct my research on the properties of stars hosting planets.

Observational astronomers like Geoff Marcy estimate the number of potentially habitable (“Earth-like”) planets in the Milky Way galaxy by extrapolating from a small number of roughly Earth-sized planets we’ve found orbiting in the Goldilocks zones around “sun-like” stars. Never mind that these three parameters are often very liberally defined. More importantly, there is a world of difference (pun intended) between “Earth-size” and “Earth-like.”

“Earth-size” versus “Earth-like”

Even if an Earth-size planet orbits near its sun’s Goldilocks zone, it may not be habitable. Just look at Venus and Mars. Venus, only a little smaller than Earth, is too close to the sun for life. Mars is both too small and too far from the sun.

Astrobiologists typically focus on these three factors — a planet roughly earth-sized, orbiting a sun-like star, and orbiting in its Goldilocks zone — because they are observable. These are all necessary factors for habitability, but they are nowhere near sufficient. There are far more details involved, and this is where things get really interesting.

In his widely discussed Wall Street Journal op-ed, published on December 25, 2014, Eric Metaxas mentions as an example one additional “local” factor for planetary habitability — a massive outer planet. Without Jupiter’s gravitational influence, the inner planets in the Solar System would experience more frequent bombardment from small bodies like comets.

If this were the only effect Jupiter had on the habitability of our Solar System, this would be significant, but it’s even more important than that.

It’s Complicated

Long-term simulations of the gravitational interactions among the planets and small bodies in our Solar System and others have revealed the complex dynamics that affect how habitable the planets will be. Terrestrial planets (rocky planets that might be able to sustain life) that form identically can later have radically different degrees of habitability if the architectures of giant planet orbits in their host planetary systems differ.

For instance, the stability of the orbits of terrestrial planets hinges on the particular properties of giant planets. Giant planets that are too massive, or too many, or too close to their host stars, or that have orbits that are too elongated can prevent smaller planets from having the long-term stable circular orbits they need to sustain life.

Giant planets also strongly influence the transport of water to the terrestrial planets during their early, formative stages. Too little or too much water can pose problems for life.

Subtle gravitational tidal interactions among the terrestrial and giant planets and the host star can perturb the terrestrial planets’ rotation axes. Include a large moon around a terrestrial planet and you might get a highly stable rotation, as with the Earth-Moon system. Recent research highlights the ways that the shape of a terrestrial planet’s orbit, the tilt of its rotation axis, its rotation period, and the way it generates heat internally are all interrelated. All these significantly influence the planet’s climate, and so affect whether it can sustain life.

Research is making it increasingly obvious that habitability depends on far more than a few planetary “ingredients” — the few we usually hear about in the breathless news stories about “Earth-like” planets.

To grasp the full picture, we have to take account of the myriad details of a planet’s origin, the way it changes over time and its present status.

What’s more, all the factors interact in complex ways that we are only beginning to understand. A small change in one of these may affect the others, resulting in a dead world.

This is just a snap shot of what is being discovered in modern astrobiology research. It is a fascinating time to be an astronomer, but we should be skeptical of the hype. We haven’t yet found any planets that are even as Earth-like as Mars — our second closest, and lifeless, planetary neighbor.

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