Earth is becoming less habitable every day, with scientists marking the year 2500 as the beginning of its demise. Around the world, many call for action with the slogan popularized by Mike Berners-Lee: there is no Planet B. But could recent spatial explorations suggest the existence of a backup planet? The system of Alpha Centauri has inspired a new hope.
Above: Alpha Centauri star system generated by images from the Digitized Sky Survey 2 in 2012. Image courtesy of Davide De Martin and Space.com.
The system of Alpha Centauri has inspired a new hope. In 150 A.D., Greek astronomer Ptolemy recorded the first observations of our closest star neighbors, the Alpha Centauri System. At a distance of 25 trillion miles from Earth, Alpha Centauri is a triple star system made up of a binary star—Alpha Centauri A and Alpha Centauri B—and a third component—Alpha Centauri C—that is currently the closest star to Earth.
Above: The triple star system of Alpha Centauri and its surrounding planets. Image courtesy of Reed Lessing.
In 1696, Father Jean Richard discovered the binary system while observing a comet. Later, in 1915, Robert Thorburn Ayton Innes discovered Proxima Centauri using two photographs taken in 1900 and 1915 and a blink comparator, an apparatus that allowed astronomers to quickly switch between photographs and view any slight changes. Innes’ study found that the third star had a similar movement to the binary system but was located two degrees to the left.
Our Sun Versus Alpha Centauri
Table 1: Characteristics of the Alpha Centauri System (Rigil Kentaurus, Toliman, and Proxima Centauri) and the Sun.
1Spectral class classifies stars based on their surface temperature, which affects the star’s elements.
2The Hertzsprung-Russell Diagram plots stars depending on their luminosity (energy radiated per second) and temperature (in Kelvin) and is used to predict their lifespan.
3☉ denotes a measurement of size in proportion to the measurements of the Sun. For example, 1.1☉ indicates that the size of Alpha Centauri A relative to that of the Sun is 10% larger.
The Hertzsprung-Russell Diagram plots stars based on their luminosities and surface temperature, helping scientists make predictions about their stellar evolution and life cycle. Both the Sun and Alpha Centauri are main sequence stars and therefore are currently undergoing the fusion of hydrogen. Unlike the others, Proxima Centauri is a considerably smaller and cooler much smaller red dwarf star. Red dwarf stars are more efficient at hydrogen fusion and have a longer lifespan than other stars. Because Proxima is also a flare star, it varies in brightness by more than one magnitude within a few minutes.
Alpha Centauri A is the most similar to our sun. Although Alpha B is similar to the Sun in terms of spectral class, size, and temperature, its orange hue indicates that it emits more x-rays than Alpha A and the Sun.
Above: Alpha Centauri in the HR-System. Image courtesy of the European Southern Observatory.
Planets of the Alpha Centauri System
As of November 2023, scientists believe five planets exist in the Alpha Centauri System: three orbiting Proxima Centauri and two orbiting Alpha Centauri B. All the planets have been discovered using the radial-velocity method, which relies on the fact that a planet’s motion creates a slight pull on the star it orbits, causing variations in light waves called wobbles. As explained by the Doppler shift, astronomers can observe changes in these wobbles to locate planets in space.
While the information on Alpha Centauri Bc is limited, Alpha Centauri Bb has been further studied but deemed inhabitable due to excessive radiation emitted from Alpha Centauri B. For comparison, Alpha Centauri Bb is closer to Alpha Centauri B than Mercury is to the Sun, making it impossible for life to survive there.
Due to their greater proximity to Earth, researchers have gathered more information on the three planets orbiting Alpha Centauri C.
Table 2: Characteristics of the Earth and Proxima Centauri B, C, and D.
1Astronomical units (AU) is a measurement designed by scientists to avoid using large numbers when describing distances in space and is the distance from Earth to the Sun.
2Eccentricity is a measure of the curvature of a planet’s orbit around its star.
Proxima Centauri B as Planet B
The only Earth-like planet is Proxima B. What evidence exists to support or dispute this claim? NASA has identified six factors for a planet to support life. The three necessary factors are the possibility of water, positive atmospheric characteristics, and sustainable temperature. The three additional—but not necessary—factors are the possibility of liquids, a constant energy source, and the building blocks of life.
Habitable Zone: A planet’s ability to maintain liquid water on its surface is determined by its position relative to the star. The Habitable Zone is the distance from a star that a planet must be to be able to support water. This zone depends on a planet’s orbit, mass, and radiative flux (the amount of radiation coming from the host star). According to NASA’s latest reports, Proxima B is in the Habitable Zone of Alpha Centauri. Scientists theorize that water may be present in the sunniest regions of the planet, yet its presence is yet to be proven.
Atmosphere: The Earth’s atmosphere—specifically the greenhouse gases and the ozone layer—protects us by blocking harmful amounts of radiation from the Sun. Proxima B may offer similar protection: although it is closer to its host star than Earth is to the Sun, Proxima Centauri is a red dwarf and therefore has a lower energy output and radiates less onto Proxima B. However, researchers have gathered little conclusive evidence about the atmospheric conditions to date. Moreover, Proxima Centauri is a flare star and therefore has bursts of radiation. One NASA study has shown that—due to radiation—Proxima B’s atmosphere is deteriorating at a much faster rate than that of Earth. This puts into question whether or not the planet could be sustainable for humans.
Temperature: Scientists have yet to prove the existence of an insulating atmosphere around Proxima B, which would allow for temperatures warm enough to hold liquids on the planet’s surface at around 86 degrees Fahrenheit. However, if experts deem that the atmosphere in Proxima B is nonexistent, the estimated temperature would be closer to -40 degrees Fahrenheit, eliminating the possibility of liquids on the surface.
Constant Energy Source: Because the red dwarf star will have a longer life cycle than Earth and will continue to fuse hydrogen for millions of years, Proxima B is in the presence of a sustainable energy source.
Distance from Earth: Although the Alpha Centauri system is our closest neighbor, it is still four light years away. Even if Proxima B were habitable by humans, current civilization lacks the capabilities to make the length trip.
Concluding Remarks
Overall, further knowledge of Proxima B’s atmospheric characteristics could strengthen its candidacy for Planet B. But one major obstacle remains—its distance from us on Earth. This realization makes it that much more important to behave sustainably, as decades remain before we succeed in developing the technologies needed to make the journey.