A recent news story reported on the discovery of two planets orbiting the red dwarf star Gliese 887 (aka Lacaille 9352). Due to their sizes, both planets were called super-earths. While the news account did not claim these planets were similar to earth, it was written in such a way that many readers might think that one or both of these planets were earthlike. Let’s delve into the details.
The researchers used the radial velocity method to detect two planets orbiting Gliese 887 (with an unlikely possibility of a third—although some popular science reports trumpeted a three super-earths find). The radial velocity method looks for small periodic Doppler motion in the spectra of stars. Due to Newton’s third law of motion, as a planet orbits a star, the star orbits the planet. It is this subtle periodic orbital motion of the star that the radial velocity method looks for. The radial velocity method results in the orbital period of the planets found. If we can infer the star’s mass, then we can determine orbital distance for the planets. In the case of Gliese 887, we have confidence that we know its mass reasonably well, so we have some certainty about the orbital sizes of the two planets.
The reason for calling them super-earths is to hold onto any vestige of these planets being like earth in some way.
The radial velocity method also measures the minimum mass of the planets found. Finding the exact mass requires knowing the inclination of the orbital plane, which we generally don’t know. The minimum mass of Gliese 887b, the inner of the two planets orbiting Gliese 887, is 4.2 ± 0.6 the earth’s mass. The minimum mass of the other planet, Gliese 887c, is 7.6 ± 1.2 the earth’s mass. Since these are minimal masses, their masses could be far higher. In my estimation, this places these two planets in the range of gas giant planets such as Uranus and Neptune, so it is misleading to call them super-earths. The reason for calling them super-earths is to hold onto any vestige of these planets being like earth in some way. Without any knowledge of the sizes or densities of these planets, it is impossible to know what kind of planets they are.
The gold standard in exoplanet discovery is finding earthlike planets. Besides size, what other requirements are necessary for classifying a planet as earthlike? Very important is orbiting in the habitable zone, the narrow region around a star where a planet with the right conditions could support liquid water on its surface. The paper explained that the habitable zone of Gliese 887 is approximately 0.19 to 0.38 astronomical units (AU, an AU is the average distance of the earth from the sun). The orbital distance of Gliese 887b is .068 ± 0.002 AU, and the orbital distance of Gliese 887c is 0.120 ± 0.004 AU. Thus, both these exoplanets orbit too closely to Gliese 887 to be in its habitable zone. The paper made note of that fact, but also mentioned that the outer planet was close to the habitable zone, apparently making a faint attempt to justify the possibility of liquid water on its surface no matter how remote that possibility might be.
The paper also computed an equilibrium surface temperature for either planet based upon an earthlike atmosphere. The inner planet’s equilibrium temperature was computed to be 468 K = 195° C = 319° F. Obviously, this planet is not earthlike. The equilibrium temperature computed for the outer planet was 352 K = 79° C = 174° F. Most people would recognize that this is far too hot to be earthlike either. By the way, the press account that I linked above misreported this latter temperature as 70°C (which would equate to 158° F). To people mostly familiar with Fahrenheit (and unfamiliar with Celsius), that incorrect temperature may not sound too high, but when converted to Fahrenheit, it is easy to see that even that low-ball temperature is far too high for this planet to be considered earthlike in the least.
So, why all the excitement about these two newly discovered exoplanets? At only 10.7 light years away, Gliese 887 is one of the closest stars. That places these planets in the neighborhood. Being so close, Gliese 887 is one of the brighter red dwarfs, around which so many other exoplanets have been found. It is hoped that its proximity may help in detecting any atmospheres these planets have, particularly once the James Webb Space Telescope goes into operation. Another point made by the paper is that Gliese 887 is very stable as compared to other red dwarfs. Most red dwarfs are known for their variability, but more importantly, also for the source of their variability. Most red dwarfs have much spot activity and strong magnetic storms that probably would strip atmospheres from any planets orbiting in their habitable zones. So, discovery of two planets orbiting around what appears to be a stable red dwarf is welcome news. Except that the planets aren’t in the habitable zone and aren’t inhabitable anyway. The reporting on this comes across as very muddled to me.
All evidence thus far leads to the conclusion is that the earth is unique.
Consequently, there is far less to this story that the hype suggests. As has been the case so often in reports of exoplanets, a discovery is made with much fanfare, but we quickly learn details that cause one to doubt that any of them are inhabitable. All evidence thus far leads to the conclusion is that the earth is unique. Since the first discovery of an exoplanet 25 years ago, we have cataloged more than 4,000 exoplanets. If most scientists 25 years ago had been asked how many earthlike planets would there be among the 4,000 exoplanets we’d find in the next quarter century, most scientists would have had optimistic counts, perhaps in the hundreds. Very few would have predicted there would be no earthlike planets in the first 4,000 exoplanets discovered. But anyone who took Isaiah 45:18 seriously knew better, for it speaks of earth’s special status.
For thus says the LORD,
who created the heavens
(he is God!),
who formed the earth and made it
(he established it;
he did not create it empty,
he formed it to be inhabited!):
“I am the LORD, and there is no other.