Is Kepler-438b the Next Earth?

Many years ago, astrophysicist Dr. Carl Sagan used to appear on The Tonight Show with Johnny Carson and claim the most outrageous idea — at least it was thought to be outrageous by most of the audience. Sagan would confidently and assuredly make the case that there must be millions, if not billions, of other planets in the universe.

Most people, including many scientists, scoffed.

At the time, the accepted number of planets was nine — those in our own solar system. Not a single other planet had been discovered orbiting another star anywhere, despite all our extraordinary observatories.


A year before Sagan died in 1996, astronomers confirmed the first planet outside our solar system to orbit a sun-like star, a huge planet that orbits the star 51 Pegasi b every four days. Since then, the discovery of other planets, called exoplanets, has multiplied so quickly that we can now be certain Sagan underestimated the number.

As of today, the Extrasolar Planets Encyclopaedia lists 1,889 exoplanets.

But last month, NASA announced that its Kepler space telescope has discovered more than 4,000 exoplanet candidates (not all will be confirmed as planets). A year ago, Kepler came up with an additional 702 newly verified planets. That batch was especially significant because unlike most previously discovered planets, these were not bigger than Jupiter and not necessarily gassy.

Most were rocky and no bigger than Neptune or no smaller than Earth.

Of the thousands detected, only eight small planets discovered so far are in habitable zones around their stars where water is likely to exist and temperatures are neither too cold nor too hot — the so-called Goldilocks zone.

In January, NASA confirmed that Kepler-438b and Kepler-442b are in the habitable zone around their stars and are rocky and are about the size of Earth. Kepler-438b is only about 1.2 times the size of the Earth, is maybe a bit warmer than Earth and orbits a red star 470 light-years away from us. Kepler-442b is 1,120 light-years away and is 1.34 times the size of Earth.

Both are at a distance from their stars that would allow liquid water.

When NASA announced Kepler’s most recent findings in January, it finally knew enough about exoplanets to make an interesting observation: If a planet is more than 1.6 times the size of Earth, it is unlikely to be a solid rocky planet like ours that you could stand on.

Many complicated factors would have to occur to make an Earth-sized planet in a habitable zone a livable place for humans, including an atmosphere that has just the right amount of oxygen (about 22%), enough gravity to keep our bones from deteriorating and enough protection from its star’s radiation such as that provided by Earth’s magnetosphere, a result of our planet’s molten metal core.

A bonus would be if the planet were tilted on its axis to provide seasons.

One of the most remarkable facts about our discoveries is that all but a few of the planet candidates discovered in the last 20 years are within a tiny part of the Milky Way galaxy near Earth. If we were comparing the universe to a large city, these exoplanets would be just outside our back door.

Carl Sagan was a big thinker, but when it came to the number of exoplanets out there, he was actually thinking too small.

To a bright future,

Stephen Petranek
for The Daily Reckoning

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Stephen Petranek

Stephen Petranek’s career of over 40 years in the publishing world is marked by numerous prizes and awards for excellent writing on science, nature, technology, politics, economics and more. He has been editor-in-chief of the Miami Herald’s prestigious Sunday magazine, Tropic, and has covered a wide range of topics for Time Inc.’s Life magazine. His...

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