The Latest “Earth-Like Planet”: LHS 1140 b

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Another Planet Found in the Habitable Zone of a Star

On April 19, 2017, a team of researchers announced the discovery of the latest “earthlike” planet, LHS 1140 b. The star the planet orbits, LHS 1140, is an M4.5 main sequence star. This red dwarf is about one four-thousandth as bright as the sun. Consequently, the star’s habitable zone, the region in which a planet must orbit for liquid water to exist on the planet’s surface, is smaller than the sun’s habitable zone. Therefore, the planet LHS 1140 b orbits its star only 9% the distance from its star that the earth orbits the sun. And rather than taking a year to orbit, LHS 1140 b orbits every 25 days.

What Happened to the Last Earth-Like Planet?

Interestingly, less than two months earlier, the world was abuzz with the last “earthlike” planet discovery when not just one but three earth-sized planets were found orbiting the star TRAPPIST-1 within its habitable zone. As I opined shortly thereafter, the star TRAPPIST-1 is prone to flares that almost certainly would make those three worlds uninhabitable. There wasn’t much mention of that fact at the time, but one of the popular accounts about the discovery of LHS 1140 b brought this up, contrasting the perceived stability of LHS 1140 compared to TRAPPIST-1. So much for TRAPPIST-1’s three earth-like planets.

Is LHS 1140 b Really Earth-Like?

But is the planet LHS 1140 b out of the woods as far as flares go? Astronomers believe that its star rotates with a period of 130 days. Rapid rotation likely is connected to flare activity (TRAPPIST-1 rotates every three days), so perhaps flaring is not a major problem with LHS 1140. However, another study indicated that over a decade the star LHS 1140 had a standard deviation in its measured brightness of about 1.2%. This variability is in addition to any dimming due to the planet’s transits. If this variability is real, any planets orbiting it would have swings in the amount of radiation they received, leading to changes in conditions on their surfaces that might be harmful to life. The nature of any variability in this star is yet unknown. We are not certain, but variability among stars of this type may be common.

What of the planet LHS 1140 b? Its diameter is 1.4 times that of the earth, qualifying it as a super earth, a planet slightly larger than the earth. The mass of LHS 1140 b is less certain, but it appears to be about 6.6 times that of the earth. Combining its mass and size, the computed density of LHS 1140 b is 2.3 times greater than the earth’s density. That is an incredible 12.5 gm/cm3. However, there is some uncertainty in the mass and size of the planet, so the lower limit on the density still would be more than 9.0 gm/cm3. The article announcing the discovery of the planet stated that its density is “consistent with a rocky bulk composition.” However, we have four planets in the solar system that have rocky bulk composition. Three of them, Mercury, Venus, and earth, have density of about 5.5 gm/cm3. This may be nearly the maximum density of a planet with rocky bulk composition.

If the computed density of LHS 1140 b is correct, then it has a strange composition. The cores of the rocky planets in the solar system are mostly iron and nickel. Those elements normally have density of 8 gm/cm3, but there is some compaction of the core. For instance, the central density of the earth’s core probably is close to 13 gm/cm3. It may not be possible to construct a plausible model of a planet with the density of LHS 1140 b without including some very dense, but cosmically rare, elements. At the very least, LHS 1140 b must not contain much rock, but instead consists almost entirely of heavier metals. With very little rock, the surface chemistry of this planet must be very different from earth’s.

There is another problem. The acceleration of gravity on the surface of LHS 1140 b is about three times that of earth’s. This is significant, because, in the solar system, planets with high surface gravity have atmospheres dominated by polyatomic gases. Polyatomic gases are greenhouse gases that trap heat from the sun. If this trend plays out with LHS 1140 b, then the surface temperature of LHS 1140 b might be too high for liquid water to exist. Neither is it likely in such an atmosphere that free oxygen exists. These facts do not bode well for this planet being earth-like.

Conclusion

A decade ago, astronomers welcomed the discovery of the first potential earth-like planet, Gliese 581 d. Almost no one seemed to notice that the star it orbited, Gliese 581, was a variable star. Even more embarrassing, there is considerable doubt that Gliese 581 d even exists. Each new supposed earth-like planet discovery since has suffered from similar problems, such as orbiting a variable star or likely undergoing synchronous rotation, probably eliminating all of them from serious consideration as being earth-like. The latest such claims about LHS 1140 b seem less problematic—we don’t yet know for certain if its parent star is a variable star. And this planet orbits a bit farther from its star than other recently claimed earthlike planets do, which makes it less likely that it rotates and orbits at the same rate. However, this planet’s size and mass are problematic. Its composition almost certainly is very different from the earth’s. Furthermore, its stronger gravity probably means that this planet has an atmosphere that is entirely wrong for living things.

Despite the problems with each supposed earth-like planet they have discovered, most scientists are undeterred, because in the evolutionary worldview life must exist on many planets. But the more extrasolar planets that we discover (the number is closing in on 4,000), the more we realize how special the earth is. This is consistent with the expectations of the creationary worldview, but it is a huge problem for the evolutionary worldview.

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