Quest for Goldilocks Planet Scores a Hit with Kepler-186f

Kepler Space Telescope pinpoints a potentially habitable earth-sized exoplanet.

Kepler-186f, the fifth planet orbiting an M1-spectral type red dwarf star in the constellation Cygnus, will go down in history as the first earth-sized extrasolar planet discovered in its star’s habitable zone. (An “extrasolar” planet, or “exoplanet,” orbits a star other than our own.) Kepler-186f has a diameter just 10% greater than earth’s, so if it turns out to have a rocky earth-like composition and if it has an atmosphere, there’s a reasonable chance it could have that all-important ingredient for life—water. But does that mean it is inhabited or even habitable? No.

To be in the “habitable zone” means to orbit its star at a distance likely to allow liquid water to exist. Planets in the habitable zone of a star may not be water-friendly or habitable, as evidenced by Venus in our own solar system. And liquid water can occur in places outside a “habitable zone” as evidenced in our own solar system by Saturn’s moon Enceladus and Jupiter’s moon Europa. But when it comes to checking out faraway extrasolar planets, the habitable zone is the most reasonable place to start looking for a watery rocky world like our own.

Just a bit larger than earth, Kepler-186f is in the habitable zone of its solar system. It orbits a cooler sun than ours every 130 days. Image: Artist’s conception from NASA Ames/SETI Institute/JPL-Caltech through NASA.gov

With an orbital period of 130 days, Kepler-186f is closer to its star than we are to ours, but its sun is smaller, fainter, and cooler. Four inner planets, Kepler-186 b, c, d, and e range from 8% to 40% larger than earth. They are too close to their star to be habitable. Answers in Genesis astronomer Dr. Danny Faulkner says, “Of the many planets found thus far, precious few have been in the habitable zones of their respective systems, so this recent discovery is of great significance.”

Sizing It Up

Gravity depends on a planet’s size and composition, which together determine mass. Without enough gravity, a planet or moon can’t hang onto an atmosphere. Our own solar system illustrates the effect of gravity. Dr. Danny Faulkner explains:

Being a companion to the earth, the moon is in the sun’s habitable zone too, but it cannot support life. There are a number of reasons why this is the case, the chief being that the moon is too small to have enough gravity to hold on to an atmosphere. With no atmosphere, liquid water is not possible. So, gravity is important, but as is so often the case, you can have too much of a good thing.

Many extrasolar planets found thus far are very large. The large planets in our solar system (Jupiter, Saturn, Uranus, and Neptune) have so much gravity that they have retained thick atmospheres made largely of hydrogen and helium, as well as a number of other gases. These gases are not conducive for life, so most astronomers think that very large planets cannot support life, even if they are in habitable zones of their respective stars.

How We Know What We Know

“The Kepler mission, for which this star and planets are named, has been a very successful program for finding planets that orbit other stars,” says Dr. Faulkner, though many are too small to hold onto an atmosphere, too large to likely have a rocky earth-like composition, or orbit too close or too far from their star to reasonably expect a temperature that could sustain liquid water.

Planets don’t shine like stars, and they are very far away. So how do astronomers see them, much less know their orbital periods? Kepler keeps a continual watch on about 150,000 stars, watching for variations in their brightness. Dr. Faulkner says:

The Kepler spacecraft has sensitive wide-field cameras that very accurately and repeatedly measured the brightness of many stars. If we observe a planetary system around another star near the orbital plane of the planets, then we will see periodic dimming of the star as the planets pass in front of the star in an event that we call a transit. Study of a transit event allows us to determine the diameter of any planets that may occult the star. The duration between successive transit events of a particular planet allows us to determine how far from the star that the planet orbits.

In the Zone

Because life thrives on earth, we look at our own solar system to define a star’s habitable zone. Dr. Faulkner explains:

How far from its star that a planet orbits is important, because it gives us a rough estimate of the possible surface conditions on a planet. Here in our solar system, the innermost two planets are so close to the sun that they are far too hot for liquid water, while the five outer planets are so far from the sun that their temperatures are too cold for liquid water. In our solar system, only on the earth is the temperature right for liquid water, an essential ingredient for life. This has led astronomers to define the habitable zone as the region around a star where liquid water is possible on a planet’s surface. Since the purpose for the search for extrasolar planets is to identify planets where life may exist, planets in the habitable zones of stars amount to the gold standard of this search.

Facts Not on File

Kepler-186f is about 500 light years away—about 32 million times farther away from us than our own sun. Is it habitable? Much remains to be determined.

Gravity depends on mass. We need to know Kepler-186f’s composition to determine that. If it has sufficient gravity to hang onto an atmosphere, Dr. Faulkner notes, the gases in that atmosphere would also affect the planet’s capacity to support life.

The ideal planet would have an atmosphere dominated by nitrogen, as is earth’s. Indeed some scientists have suggested looking for spectral signatures in the atmospheres of extrasolar planets as a way of finding truly habitable planets. It appears that planets much more massive or less massive than the earth could not have such an atmosphere. This is why Kepler-186f, only slightly larger than the earth, is so important.

The authors of the Kepler-186f discovery paper considered a wide range of possible compositions to determine what the minimum and maximum mass of that planet might be. On the low end was an ice/water composition that yielded a mass of 32% of the earth. This mass would be insufficient to maintain a significant atmosphere, so life would not be possible on Kepler-186f if this were the true mass. The maximum mass of 377% of the earth’s mass would result if the composition were mainly iron. With this much mass, Kepler-186f almost certainly would have an atmosphere, though it isn’t clear what composition that atmosphere would have.

Moving in the Zone

From earth we never see the back side of the moon. The reason for that is that the moon rotates at the same rate it orbits the earth. This is often true for lower mass objects orbiting near larger mass objects, and it’s associated with tidal forces generated by their interactions. This factor could also affect the habitability of Kepler-186f. Dr. Faulkner explains:

The authors of the discovery paper briefly discussed one aspect of the orbit of Kepler-186f that could be problematic. The star Kepler-186 is smaller and cooler than the sun, so its habitable zone is much closer to it than the sun’s habitable zone is to the sun. When a much lower mass body orbits closely to a much more massive body, the tidal forces raised by the more massive body tends to cause the lower mass body to have synchronous rotation. That is, the smaller mass body rotates at the same rate that it orbits, causing one side of the smaller body to face the larger mass body.

This is true for most of the natural satellites of the planets in the solar system, including the Earth’s moon. It is likely that Kepler-186f has synchronous rotation. If so, there is question whether this planet is habitable, because one side of the planet will remain hot while the other side will remain cold.

Old Faithful Versus Very Variable

Our own sun, for all its sunspots and solar flares, is a faithful and reliable supplier of safe energy for the world God gave us. A small variation in its overall energy output would be catastrophic. Red dwarfs, a class of stars that make up about 70% of the stars in the Milky Way, are particularly temperamental, however. That could put a wrench in the hopes of finding our galaxy rich in habitable planets! Dr. Faulkner explains:

Another concern that isn’t often raised in discussions of habitable extrasolar planets is the variability of the parent stars. For instance, Gliese 581d, and, if it exists, Gliese 581g, are within the habitable zone of the star Gliese 581. Rarely mentioned is that Gliese 581 is a BY Draconis type variable. BY Draconis variable stars can change brightness by more than 100%.

Gliese 581 varies by at most 1% at this time, but we have observed this star for only a few decades —we don’t know what its long-term behavior might be. Still, it is generally thought that a 1% change in solar brightness would be catastrophic for the earth, so why should this relatively modest variability in Gliese 581 be any different for its planets?

BY Draconis stars probably vary because of numerous spots on their surfaces. Spots probably are associated with strong magnetic fields, which in turn probably produced powerful flares and magnetic storms. Far more active than BY Draconis variables are UV Ceti stars. UV Ceti stars sometimes are called flare stars, because they are subject to frequent flares that can dramatically change their brightness in just minutes. A planet orbiting such a star almost certainly would be sterile, regardless of how good the conditions on the planet might be otherwise.

Red dwarf stars, such as Gliese 581 and Kepler-186, are more susceptible to these kinds of variability, because they have very deep convective regions that churn up their magnetic fields. We haven’t observed red dwarfs long enough to ascertain whether just some red dwarfs are active all the time or if all of them undergo such activity at different times. If the latter is true, then no red dwarf star would be a suitable candidate for hosting habitable planets.

Boldly Seeking out Where Life Could Have Evolved

In the search for alien life, it makes sense to look first at earth-like places. Evolutionary scientists expect life could evolve wherever conditions are right.1 But if life, or evidence of past life, is ever found on Mars or on some distant extrasolar planet, will that be the ultimate proof of molecules-to-man evolution? Not at all. It would demonstrate the existence of life but not that such life evolved by natural processes from lifeless chemicals.

The Bible does not say whether God created life elsewhere, but the Bible does tell us God created all life on earth in the same week He created the stars. Indeed, “without Him [Jesus Christ] nothing was made that was made” (John 1:3, NKJV).

As the search for exoplanets progresses, evolutionary beliefs about the origins of solar systems are repeatedly challenged by unexpected discoveries. Meanwhile, the search for a habitable one continues. Dr. Faulkner points out:

To date, nearly 2,000 exoplanets have been found. Only a very few have even a chance of sustaining life. However, we don’t know much about the few possibly habitable planets. One must assume many favorable things about these planets in order for them actually to be habitable. And even if a planet had the right conditions for life, our best operational science on the question of the origin of life tells us that abiogenesis (life coming from non-life) is impossible. The data thus far strongly support the contention that life is unique to the earth.

So far, it appears there is no place like home. Read more about the uniqueness of the earth God created for us in “Just Right for Life”