Centauri

November 2023
Hamilton, New Zealand

Despite the vast expanse that is our solar system, a potential infinite number of star systems lie beyond its border. So far, the furthest recorded star is Earendil, which lies 28 billion light years away (meaning that light, which travels at 300,000 km per second, takes 28 billion years to travel to or from Earendil) (1). Serious distance.

Our nearest star system is Alpha Centauri, a triple star system that consists of three stars - Rigil Kentaurus (Alpha Centauri A), Toliman (Alpha Centauri B), and Proxima Centauri (Alpha Centauri C) (2). The Alpha Centauri system is just over 4 light years away, which is nothing compared with Earendel, but still serious.

​Rigil Kentaurus and Toliman form a close-knit binary star system that has the appearance of a single star in the night sky, which has allowed it to be labelled as the third-brightest "star" in the sky after Sirius and Canopus. Rigil Kentaurus and Toliman orbit around a common center, and each of them is similar in size and luminosity to our own sun. Yet while these two stars are what we see of Alpha Centauri in the sky, they are not the most intriguing component of this star system.

The third star in the system, Proxima Centauri, is much more interesting. Proxima Centauri is a red dwarf, so it has only 12.5% of the sun's mass and an average luminosity that is so low that the star cannot be seen by the naked eye (3). Proxima Centauri is also 4.25 light years away, which makes it the closet star to the Earth. However, the most interesting aspect of this star relates to the several planets within its orbit.

The "star" on the left is actually the combined light of two stars, Rigel Kentaurus and Toliman.

Shot of ​Proxima Centauri, taken by Hubble.

Planets and Life

As far as we know, there are three planets in orbit around Proxima Centauri. There are two confirmed planets, which are (unimaginatively) named Proxima Centauri b and Proxima Centauri d. There is also an unconfirmed planet, Proxima Centauri c, the existence of which remains controversial.

Discovered in 2016 (4), Proxima Centauri b is the most exciting of the three planets. First, it has a similar mass compared with Earth (5). Despite the similar mass, the composition of Proxima Centauri b may differ from that of Earth's - it could be a much smaller, denser planet than Earth, or it could be a relatively large, watery planet. Second, despite Proxima Centauri b being roughly 20 times closer to its star than Earth is to its sun, the low luminosity of Proxima Centauri means the surface temperatures of Proxima Centauri b are relatively hospitable, averaging about -40 degrees Celsius. In fact, Proxima Centauri b lies within the "habitable zone" of Proxima Centauri, which means this planet can support liquid water and potentially, life.

The existence of Proxima Centauri d was confirmed in 2022 (6), with a mass about 25% that of Earth (7). This planet lies much closer to Proxima Centauri, leading to an average surface temperature of about 90 degrees Celsius, which is too hot for most forms of life as we know them. However, it is thought that the polar regions might be cooler and therefore possess habitable temperatures.

The existence of Proxima Centauri c remains disputed, but if it does exist, it is probably a much larger planet, with a mass about 7 times that of Earth (8). Moreover, given its larger distance from Proxima Centauri, Proxima Centauri c would have surface temperatures of around -235 degrees Celsius, which is not compatible with life.

Artist's impression of Proxima Centauri b from space.

Comparison of Earth with Proxima Centauri b.

Artist's impression of Proxima Centauri b from the surface.

Explorations

Although Alpha Centauri is our closest star system, it is still well over 4 light years away, meaning that any spacecraft reliant upon conventional methods of propulsion would require many thousands of years to get there. For example, the satellite Voyager 1 is travelling at a blistering 60,000 km per hour, but even at this speed Voyager 1 would still take an incredible 74,000 years to reach Alpha Centauri. If we are to ever reach this star system, non-conventional methods will be required.

Perhaps surprisingly, the last 70 years or so have seen a number of initiatives aimed at reaching Alpha Centauri (9-11). The first, Project Orion, was formed in the 1950s and 1960s. Orion aimed to build a "nuclear pulse" spacecraft that would be propelled by a series of atomic explosions behind it, which would impact against a special plate and jettison the ship to a velocity of 10,000 km per second (3.3% the speed of light). Such a spacecraft could travel to Alpha Centauri in perhaps 133 years. Next, in the 1970s, came Project Daedelus (which was actually designed to reach Barnard's star, 5.9 light years away from Earth). The goal of Daedelus was to create a huge ship that would be propelled by a fusion rocket, launching the ship to 36,000 km per second (12% the speed of light). The ship was to be partially fueled by gases mined from the planet Jupiter and would contain a number of robot wardens that could repair it as needed. Later on, in the 1980s, came Project Longshot. The goal of Longshot was to build a ship powered by a nuclear fission reactor, which would power a bunch of lasers that could ignite a fusion rocket, similar to Daedelus.

In 2016, the latest proposal, Project Breakthrough Starshot, was developed by Yuri Milner, Stephen Hawking, and Mark Zuckerberg (12). The premise of Breakthrough Longshot differs dramatically from its predecessors in that it is based around a fleet of about 1,000 light-driven nano-spacecraft, each of which would weigh just a few grams. Lasers on Earth's surface would propel each spacecraft one at a time, and the large number of ships would ensure that enough would survive the journey to capture images on their cameras and the send the data back to Earth. Project Breakthrough Starshot aims to propel the nano-ships to a crazy velocity of up to 30,000 km per second (up to 20% the speed of light).

Project Orion aimed to build a "nuclear pulse" spacecraft propelled by atomic explosions, which could travel at 3.3% the speed of light.

Project Daedelus aimed to build a spacecraft propelled by a fusion rocket, which could travel at 12% the speed of light.

Project Longshot aimed to build a spacecraft powered by a fission reactor, which could also power a fusion rocket.

Regardless of the success of Breakthrough Starshot or any future projects, the whole point of interstellar travel is to ultimately facilitate human exploration of the planets of Alpha Centauri, as well as those of further star systems. This means that we will have to develop super-fast spacecraft capable of carrying humans while travelling at least 20% the speed of light, or super-large spacecraft capable of sustaining several generations of humans over centuries of travel. Perhaps another way will open up, one not yet foreseen...it's difficult to predict.

Importantly, before we actually make any concerted effort to reach Alpha Centauri, we will probably need a good reason to do so, one that makes sense for humanity as a whole.

Not sure we have that yet. Maybe someday, we will.

​Solace.

References
(1)  https://science.nasa.gov/missions/hubble/record-broken-hubble-spots-farthest-star-ever-seen.
(2) https://en.wikipedia.org/wiki/Alpha_Centauri.
(3) https://en.wikipedia.org/wiki/Proxima_Centauri.
(4) Anglada-Escudé, G, Amado, P, Barnes, J, et al. A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536, 437–440 (2016).
(5) https://en.wikipedia.org/wiki/Proxima_Centauri_b.
(6) Faria, JP, Mascareno AS, Figueira P, et al. A candidate short-period sub-Earth orbiting Proxima Centauri. A&A 658, A115 (2022).
(7) https://en.wikipedia.org/wiki/Proxima_Centauri_d.
(8) https://en.wikipedia.org/wiki/Proxima_Centauri_c.
(9) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion).
(10) https://en.wikipedia.org/wiki/Project_Daedalus.
(11) https://en.wikipedia.org/wiki/Project_Longshot.
(12) https://en.wikipedia.org/wiki/Breakthrough_Starshot.