Proxima Centauri b: An Earth-Like Planet?

A little more than two decades ago, astronomers discovered the first known exoplanets, planets orbiting other stars. The current exoplanet tally is about 3,500, but that number is sure to grow. The impetus for the search for exoplanets is to find planets on which life is possible. The vast majority of extrasolar planet discoveries clearly are hostile to life. However, from time to time there are reports of the discovery of a possible earth-like planet. This is the situation of Proxima Centauri b, announced on August 24, 2016.

The red dwarf Proxima Centauri is the star closest to the sun. It is a distant orbiting companion to the erstwhile closest star, Alpha Centauri. While Alpha Centauri is the third brightest star in the sky (visible south of 29 degrees north latitude), Proxima Centauri is very faint. Proxima Centauri is about 10,000 times fainter than Alpha Centauri, about one hundred times fainter than the faintest star visible to the unaided eye. When astronomers find a planet orbiting another star, they give the planet the designation of the star, followed by the lower-case letter b, hence the name of this particular planet. Any subsequent planetary discoveries around the same stars are given sequential lower-case letters in order of discovery.

What qualifies an extrasolar planet as being earth-like and hence a possible haven for life?

What qualifies an extrasolar planet as being earth-like and hence a possible haven for life? First, a planet must orbit in a star’s habitable zone. The habitable zone is the narrow region around a star in which the possibility of liquid water, thought essential for life, can exist. If a planet orbits its star closer than the habitable zone, the planet’s surface likely is too hot for liquid water to exist. If the planet orbits farther away, the planet’s surface probably will be too cold for liquid water. The distance of the habitable zone from a particular star depends upon the star’s temperature and brightness.

While being in the habitable zone is a necessary condition for life, it is not a sufficient condition. A planet also must have the proper kind of atmosphere. Planets that are too small lack gravity to hold on to much of an atmosphere. This is the situation of Mercury, Mars, and the earth’s moon. Without a significant atmosphere to provide pressure that can contain water, liquid water cannot exist. But if a planet is too large, its much greater gravity tends to hold onto the wrong kind of atmosphere. This is the situation of Jupiter and the other three Jupiter-like planets in the solar system. What constitutes a wrong atmosphere? There are several ways that an atmosphere can go awry.

Some gases are directly hostile to life. If they are in abundance, polyatomic gases can be harmful indirectly. Polyatomic gases have three or more atoms in their molecules. Polyatomic gases block infrared (IR) radiation. IR radiation sometimes is called heat radiation, because many objects cool by emitting IR radiation. For instance, at night the ground emits IR radiation to lose heat that it absorbed from the sun during the day. Polyatomic gases block IR radiation, preventing this cooling. This is similar to how a greenhouse holds in heat, so polyatomic gases sometimes are called greenhouse gases in this context. Water vapor is the most significant greenhouse gas in the earth’s atmosphere. That is why the temperature remains warm on humid nights, but the temperature can plunge during nights when the humidity is low. Carbon dioxide (CO₂) is another greenhouse gas that can hold in heat. This is the basis for concern about global warming and climate change due to increased output of CO₂ by human sources since the industrial revolution. The planet Venus has an atmosphere that is much denser than the earth’s atmosphere, and its atmosphere is dominated by CO₂. This results in an extremely hot surface temperature on Venus. Clearly, a planet similar to Venus is hostile to life.

Contrast this to earth’s atmosphere that is dominated by diatomic gases, gases having two atoms per molecule. The major component (78%) of earth’s atmosphere is nitrogen (N₂). This gas is inert, merely providing bulk to the atmosphere. Much of the remainder of the earth’s atmosphere (21%) is oxygen (O₂), the substance that is essential for human and animal life. Greenhouse gases make up far less than 1% of the earth’s atmosphere. This small amount of greenhouse gases is ideal in that it holds in some, but not all, heat at night. This provides a modestly warm, but not hot, atmosphere. Astrobiologists, scientists who study the possibility of life elsewhere in the universe, recognize the ideal nature of the earth’s atmosphere. They reckon that the best hope for finding life elsewhere is on a planet with an atmosphere similar to earth’s atmosphere.

If a planet orbits in the habitable zone of a star, but is too small to have any significant atmosphere, it is deemed non-earth-like. On the other hand, if a planet orbiting in the habitable zone of a star is too massive, it almost certainly will have an atmosphere similar to Jupiter or perhaps even Venus, and it too is deemed non-earth-like.

How does the new exoplanet Proxima Centauri b stack up?

How does the new exoplanet Proxima Centauri b stack up? As previously mentioned, it orbits in Proxima Centauri’s habitable zone. However, the star Proxima Centauri is much smaller, less massive, and cooler than the sun. Hence, its habitable zone is much smaller than the sun’s habitable zone. Proxima Centauri b orbits just 1/20 the earth’s distance from the sun. Rather than orbiting once each 365 days as the earth does, Proxima Centauri b’s orbital period is a mere 11.2 days. The minimum mass of the planet is 1.3 times that of the earth. Since this is a minimum mass, the actual mass could be greater. This mass range almost assures that Proxima Centauri b has an atmosphere. If Proxima Centauri b’s mass is close to the minimum mass, then there is some chance that its atmosphere may have the properties similar to earth’s atmosphere, but this is not guaranteed.

But even if Proxima Centauri b has an atmosphere with composition similar to earth’s atmosphere, there are other problems. Orbiting so closely to its star, Proxima Centauri b is expected to experience tidal locking so that it rotates synchronously. That is, the planet probably orbits with one side facing Proxima Centauri. The side of the planet that always faces the star is probably far too hot for living things, while the side that is perpetually in darkness is likely too cold. Only in a ring near where the star is always up but not too high above the horizon could there be conditions suitable for life.

Depending on how planetary magnetic fields are generated, tidal locking might have dampened any nascent magnetic field that Proxima Centauri b had. This is significant, because red dwarfs like Proxima Centauri are prone to harmful radiation. The earth’s magnetic field protects the earth’s atmosphere from the flow of charged particles from the sun (the solar wind). Without this protection, charged particles from the sun would eventually strip earth of its atmosphere. The amount of the solar wind is directly related to the strength of the sun’s magnetic field. For instance, flares and coronal mass ejections (both related to the sun’s magnetic field) greatly increase the solar wind. Presumably, a star’s wind is related to its magnetic field too. Proxima Centauri’s magnetic field is hundreds of times stronger than the sun’s magnetic field, suggesting that its stellar wind is far greater than the solar wind. Red dwarfs, such as Proxima Centauri b, are prone to flares and probably experience coronal mass ejections greater than the sun does. Furthermore, being only 1/20 as far from its star, for a given level of stellar wind, Proxima Centauri b would experience 400 times as much damage as the earth does. Therefore, even with some protection of stripping by stellar wind from any magnetic field that it might have, Proxima Centauri b probably cannot protect its atmosphere. So even if Proxima Centauri b initially had an atmosphere, it probably lost it. Without an atmosphere, life if not possible. Finally, the increased level of activity of the star Proxima Centauri and Proxima Centauri b’s close proximity to it likely causes the planet to experience far higher levels of ultraviolet and X-ray fluxes than the earth does. These radiations are harmful to life.

It doesn’t take very long to realize that each of these supposed earth-like planets have severe problems that reveal that they are anything but earth-like.

Evolutionists understandably are excited each time an extrasolar planet, such as Proxima Centauri b, shows any possibility of being earth-like. In their worldview, there is nothing special about the earth, because if the earth is special, that suggests design. Therefore, the vast majority of evolutionists assume that there must be many earth-like planets, with life abounding in many places. However, it doesn’t take very long to realize that each of these supposed earth-like planets have severe problems that reveal that they are anything but earth-like. What do biblical creationists expect? In the creation account of Genesis 1, we see God taking great care to create a world for man’s habitation. This is explicitly restated in Isaiah 45:18. From this we conclude that the earth truly is unique and that there are no earth-like planets. We find that the best science available agrees with this.