Alien Skies: The View from Alpha Centauri

Cover of the 1970s paperback edition of Fact and Fancy which contains Isaac Asimov’s essay, “The Planet of the Double Sun”.

Growing up in the late 1960s and 1970s, I was an avid viewer of science fiction on television. Naturally, the programs I watched included the classic series Lost in Space where I learned the name of the first star I knew other than the Sun – Alpha Centauri (also written as α Centauri) which was the destination of the Robinson family flying on board the Jupiter 2. As a young budding astronomer in the early 1970s, I learned that α Centauri was the closest star system to the Sun and appeared as the third brightest star in our nighttime sky with a V magnitude, m_V, of -0.27 (unfortunately, I could not see it from my home in New England). I also learned that it was actually a multiple star system composed of a pair of Sun-like stars and a dim red dwarf called Proxima Centauri, because it was a fraction of a light year closer to us than the other stars.

As I entered my teens in the mid-1970s, I became a voracious reader and sought out books about the nearby stars, the possibility of exoplanets and interstellar travel, among other topics. My favorite author at this time was Isaac Asimov, not for his science fiction, but for his essays on science. Around 1976 I read “The Planet of the Double Sun” in one of the numerous paperback collections of Asimov’s science essays I owned. In this essay, Asimov wrote about how the sky would appear if instead of orbiting the Sun, the Earth orbited inside a double star system. In order to simplify his calculations, Asimov used an idealized model loosely based on α Centauri but it painted my first mental image of what the view from an Earth-twin orbiting α Centauri would be like. With decades of additional experience (including a degree in physics) as well as much more detailed information available about the α Centauri system, I was curious about the actual appearance of the sky from α Centauri and decided to revisit the question for myself.

Background

The α Centauri system is the closest known to the Sun and consists of three stars. On November 6, 2016 the International Astronomical Union (IAU) officially adopted the ancient name for this star, Rigel Kentaurus – a Latinization of the Arabic name رجل القنطورس or Rijl al-Qanṭūris meaning “foot of the centaur”. At the heart of this system are a pair of Sun-like stars 4.37 light years from us that are locked in orbit around each other. The larger component of this pair, α Centauri A, is a G2V star like the Sun while the smaller component, α Centauri B, is a slightly cooler and dimmer K1V star. Estimated to be around 5 billion years old, these two stars are among the most Sun-like in our neighborhood.

This diagram shows the relative sizes of the Sun compared to the stars of the α Centauri system. Click on image to enlarge. (PHL)

About 2.2° to the southwest of this pair of stars as viewed from Earth is α Centauri C better known by its official IAU name (adopted on August 21, 2016) of Proxima Centauri because, at a distance of 4.24 light years, it is the closest main sequence star to our solar system. Unlike α Centauri A and B, Proxima Centauri is a very small and intrinsically dim M5.5V red dwarf. The properties of these three stars are summarized in Table I below.

Table I: Star Properties

While α Centauri was known since ancient times by those living in or travelling to the southern hemisphere, it was not until 1685 that Jesuit missionaries in Africa made the first telescopic observations of this star and discovered that it was in fact a pair of stars. Careful observations over the following centuries showed that α Centauri A and B are locked in a moderately eccentric orbit about each other with a period of 79.92 years, a periastron separation (i.e. the closest point in their orbit) of 11.28 AU (astronomical unit defined as 149,597,870.7 kilometers) and an apastron distance (i.e. the farthest point in their orbit) of 35.81 AU. These distances range roughly from the orbit of Saturn to that of Neptune in our own Solar System.

A diagram showing the apparent and actual trajectory of α Centauri B. Click on image to enlarge. (Wikimedia Commons/SiriusB)

Because of its close proximity to α Centauri AB and its shared apparent motion, it has been generally believed since its discovery in 1915 by Scottish astronomer Robert Innes that Proxima Centauri orbits the pair of larger, Sun-like stars. The latest measurements indicate that Proxima Centauri is currently about 12,900 AU from the barycenter of α Centauri AB – a distance equivalent to about 75 light days. Increasingly detailed measurements of the relative positions and motions of these stars has shown that Proxima Centauri is indeed gravitationally bound to α Centauri AB in an orbit which ranges from 5,300 AU to 12,900 AU with a long period of around 591,000 years (see “The Orbit of Proxima Centauri”). Proxima Centauri is currently near its apastron and will soon start closing its distance with α Centauri AB over the next third of a million years. Details of the orbits of the stars in the α Centauri system can be found in Table II below.

A 3D diagram showing the orbit of Proxima Centauri around α Centauri AB. Click on image to enlarge (Kervella et al.)

Table II: Star Orbits

Being the closest star system, the stars of α Centauri have been targets for numerous searches for exoplanets over the past several decades with sometimes disappointing results (“The Search for Planets Orbiting Alpha Centauri – II”). The most recent of these was the announcement in 2012 of an Earth-size exoplanet locked in a 3.2-day orbit around α Centauri B (see “The Search for Planets Orbiting Alpha Centauri”). After numerous failed attempts to confirm this discovery, in 2015 it was found that the signal in the precision radial velocity measurements corresponding to the exoplanet α Centauri Bb was instead an artifact created by how the radial velocity was sampled over time and that this exoplanet did not in fact exist (see “Alpha Centauri Bb Five Years Later – The Search for Exoplanets Continues”). The search for bona fide exoplanets orbiting α Centauri A and B continues.

This image, purportedly showing Proxima Centauri c, was created by stacking five images taken in April 2018 using VLT-SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) located at European Southern Observatory on Cerro Paranal in Chile. The yellow circles denote the planet’s position in April 2018 and its expected position in April 2020 with the dashed blue line indicating the best fit orbit. Click on image to enlarge. (Gratton et al.)

Astronomers have had much better luck in their search for exoplanets orbiting Proxima Centauri. In 2016, the discovery of an exoplanet was announced which was found using precision radial velocity measurements. With a minimum mass or M_Psini currently pegged at 1.17 Earth masses (M_E), the roughly Earth-mass Proxima Centauri b was found to be orbiting inside the habitable zone (HZ) of this closest main sequence star (see “Habitable Planet Reality Check: Proxima Centauri b”). Subsequent analysis of an expanded radial velocity data set showed signs of a still more distantly orbiting exoplanet called Proxima Centauri c (see “Proxima Centauri b One Year Later: The Search for More Exoplanets Continues“). Combining these data with precision astrometric measurements and images purportedly showing Proxima Centauri c has allowed astronomers to determine that it has an orbital period of 5.3 years and a mass of 7 M_E, using the derived orbit inclination of 133°. Assuming that the exoplanets of the Proxima Centauri system have coplanar orbits, this implies an actual mass of about 1.6 M_E for Proxima Centauri b. The known properties of the exoplanets orbiting Proxima Centauri are shown below in Table III.

Table III: Planets of Proxima Centauri

The View of α Centauri B from α Centauri A

Just as Asimov had done in his essay decades ago, my calculations that follow will focus on the appearance of the sky from a hypothetical Earth-twin orbiting α Centauri A. I have assumed that this Earth-twin orbits α Centauri A at a mean distance of 1.23 AU so that its mean stellar flux (i.e. the amount of energy the planet receives from its sun) is precisely Earth’s value. From this perspective, α Centauri A would have an apparent diameter of 31.8 arc minutes and a m_V of -26.7 – essentially identical to the appearance of the Sun in our sky. This is not unexpected since α Centauri A is the same spectral type as the Sun and has essentially the same surface temperature as the Sun.

The differences between Earth and this hypothetical Earth-twin orbiting α Centauri A would become apparent over time, however. Because of its more distant orbit (owing to α Centauri A having a luminosity 50% greater than the Sun), the α Centauri A Earth-twin would take about 476 Earth days to complete one orbit. As a result, each season on this hypothetical Earth-twin would last about a month longer than they do here on Earth. Assuming that the sidereal day of this Earth-twin is identical to Earth’s, the mean solar day on the Earth-twin would be almost a minute shorter than here on Earth as a result of this slower orbit.

While these differences are subtle, even a casual observer would immediately notice the bright α Centauri B in the sky of this Earth-twin – something that is missing from our sky. From α Centauri A, α Centauri B would have a m_V ranging from -18.0 at apastron to as bright as -20.9 at periastron 40 years later. This is about 130 to 1,800 times the brightness of a full Moon here on Earth and would provide an illumination level comparable to that on Earth during a very cloudy day. And unlike the Moon with its light spread over a disk about 30 arc minutes across, this illumination would come from a tiny disk 0.8 to 2.8 arc minutes in diameter. Looking directly at α Centauri B even from this distance would probably cause eye damage and it would surely be visible even in daylight. A naked eye observer using a suitably dark piece of smoked glass would just be able to discern α Centauri B as a tiny disk (particularly near periastron) and could observe the star even as it approaches the disk of α Centauri A at opposition.

The Appearance of Any Planets Orbiting α Centauri B

What about any planets orbiting α Centauri B? We do not know about the existence of any such planets yet but, if there are any, they likely will be visible in the nighttime sky. Assuming for the moment that α Centauri B has its own Earth-twin in an orbit with a mean radius of 0.71 AU (where it would have the same effective stellar flux as the Earth), it would appear as a point of light with an m_V as bright as +4.0 to +1.2 at apastron and periastron, respectively. Unfortunately, this Earth-twin orbiting α Centauri B would have this brightness at “full” phase when its apparent distance from α Centauri B is too small to be easily seen. But at greatest elongation (i.e. when the apparent distance between α Centauri B and its Earth-twin is at a maximum as viewed from α Centauri A), where the Earth-twin is 1.1° to 4.0° from α Centauri B, the Earth-twin at half phase would still have a m_V of +2.7 to +5.5 at periastron and apastron, respectively. While it would be tough to spot this Earth-twin when α Centauri B is near apastron, it would be visible to the naked eye closer to periastron. With an apparent diameter of 0.5 to 1.7 arc seconds, it would take a moderate size telescope and good seeing conditions to notice it as a disk or its phases as it orbits α Centauri B.

If α Centauri B had planets orbiting closer in, they would be much brighter and potentially more observable. In 2013, NASA’s Hubble Space Telescope was used to search for possible transits of the now debunked α Centauri Bb. While it failed to find a transit of α Centauri Bb, Hubble did observe a single transit-like event consistent with a planet with a radius of about 0.9 times that of Earth (R_E) in an orbit with a period less than about 20 days (corresponding to a maximum orbit radius of about 0.14 AU – see “Has Another Planet Been Found Orbiting Alpha Centauri B?”). IF we assume that this exoplanet is real, it would have a greatest elongation as large as 48 arc minutes at periastron (about half again the apparent diameter of the Moon as seen from the Earth) when it would have an m_V of -1.3, assuming its reflective properties are similar to that of Venus. While it would be a tough target, it would be observable with the naked eye as it flited around α Centauri B once every three weeks.

The View of Proxima Centauri

While α Centauri B and its system of planets would dominate the nighttime sky as seen from an Earth-twin orbiting α Centauri A, Proxima Centauri would be notable by its unremarkable appearance in the sky. At its current distance of 12,900 AU, Proxima Centauri would have an m_V of only +4.5 with literally hundreds of stars in the sky that are brighter. Even when it reaches periastron about 300,000 years from now, Proxima Centauri would still have an m_V of +2.6 with several dozen brighter stars visible in the nighttime sky including the Sun (which, with an m_V of +0.5 visible in the constellation of Cassiopeia, would be on the top ten list of brightest stars as view from α Centauri). Given its small size and great distance from α Centauri A, Proxima Centauri currently would have an apparent diameter of only 23 milliarc seconds (mas) – barely observable as a disk even with the largest telescopes on Earth today. Combined with a slow proper motion averaging just two arc seconds per year (superimposed on a parallax wobble with a semiamplitude of 20 arc seconds at its current distance as the Earth-twin orbits α Centauri A), Proxima Centauri would be an unremarkable star in the sky.

Given the unremarkable appearance of the star itself, the planets currently known to be orbiting Proxima Centauri would be even less remarkable. At its current distance from α Centauri A, Proxima Centauri c would appear to have a greatest elongation of 24 arc seconds. Assuming that this planet has a density and photometric properties like Neptune (a fair approximation given what little we know about this world), Proxima Centauri c would have an incredibly dim m_V of +27 as seen from α Centauri A and a bit brighter at fuller phases. The situation with the more Earth-like Proxima Centauri b would be a bit better. At maximum elongation, this world would have an apparent separation of just 0.8 arc seconds but an m_V of +22, assuming it has an Earth-like composition and photometric properties. These worlds, as viewed from an Earth-twin orbiting α Centauri A, would be barely detectable using the technology that has become available to us during the last three decades or so even though it is just 0.2 light years away.

A Connection with Greek Mythology?

In his essay, “The Planet of the Double Sun”, Isaac Asimov speculated about the effects of having the Earth orbiting α Centauri A with a bright α Centauri B in the sky. Asimov made a connection to the annual conjunction of α Centauri A and B as seen from the former’s Earth-twin and the myth about Prometheus. In a nutshell, Prometheus took pity on the newly created humans living in the cold and dark on Earth and brought fire to them against the will of Zeus. As punishment, Zeus had Prometheus chained to a rock at the ends of the Earth where a vulture would tear his liver out every day leaving him for the night so that the organ would magically grow back only to start the cycle of torment all over.

The Torture of Prometheus by the Italian Baroque painter, Salvatore Rosa (1646-48).

Asimov proposed that α Centauri B as seen in the sky of α Centauri A’s Earth-twin would represent Prometheus. Once every year, the bright star named “Prometheus” would flee from the sun carrying fire to light the night sky for the inhabitants of the α Centauri A Earth-twin. Any prominent planet orbiting α Centauri B would become visible as the star Prometheus escaped from the sun and would represent the vulture. “Vulturius”, as Asimov proposed this planet would be named, would be seen alternately moving away from Prometheus then swoop back in every few days to a few months (depending on the planet’s orbital period) to torment Prometheus only to leave him again. As the star Prometheus left the night sky after almost a year bringing darkness back to the night, daytime observers would see the star (with Vulturius now absent from view because of the daytime sky brightness) moving swiftly towards the sun to steal fire to light the nighttime sky all over again.

Asimov further speculated about the effects on ancient Greek science of having a companion like α Centauri B visible in our sky. One of the popular Greek models of the universe, embraced in the 4^th Century BC by Aristotle, had the Earth motionless in the center with the Sun, Moon and planets orbiting around it. Around 280 BC, Aristarchus and Samos suggested that only the Moon orbited the Earth while the Earth and other planets circled the Sun at the center of the universe. Ultimately, the geocentric model won out and was refined in the 2^nd century AD by Ptolemy to be accepted in the West for another 1,500 years. But if we had an α Centauri B in our sky with planets clearly circling it, the weight of evidence may have swung in favor of a sun-centered model instead millennia earlier than it actually happened.

An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)

What If…

I would like to take Asimov’s speculation a little further into our history assuming that Earth was actually the hypothetical Earth-twin discussed earlier orbiting α Centauri A. What would we have learned and when? Asimov posited that the adoption of the Sun-centric model may have led to the discovery of Kepler’s laws of planetary motion and Newton’s law of gravitation much sooner. Assuming for the moment it did not and our history unfolded largely as it did, we would start learning more about α Centauri B and its planets almost immediately after the invention of the telescope. Galileo would have easily demonstrated that α Centauri B is a star just like our sun with a clearly visible disk up to 2.8 arc minutes across possibly sporting starspots just like our Sun.

While Galileo would not have had a Jupiter with moons to discover (any orbits larger than a couple of AU become unstable because of the presence of α Centauri B), he might have been able to observe a moon orbiting one of the planets of α Centauri B. If α Centauri B had an Earth-twin orbited by a Moon-twin, for example, it would appear as much as 20 arc seconds apart with an m_V around +9.7 near the periastron of α Centauri B, depending on the Moon’s phase. But if Galileo missed it, astronomers from later in the 17^th century likely would have found it. And around this same time into the beginning of the 18^th century, telescopes would have advanced to the point to resolve the tiny disks of the planets orbiting α Centauri B showing them to be worlds just like those orbiting α Centauri A.

The situation with Proxima Centauri in this hypothetical timeline does not make any progress until well into the 18^th century when the first detailed maps of the sky were being compiled using telescopes. Some lucky astronomer might have noticed something amiss when an otherwise unremarkable red m_V +4.5 star appeared displaced by about a half a degree from its ancient cataloged position as a result of 1,500 years of proper motion. Failing that, it would only have been a matter of time before around 20 arc seconds of “noise” was found in the position determinations of this star not present in other stars. This would have led to the first detection of stellar parallax to explain the “noise” up to a century earlier than it was first measured by us. With the discovery that Proxima Centauri was a nearby star, detailed studies would surely have started. But it probably would not have been until the late 20^th century before Proxima Centauri c was discovered by direct imaging or astrometry and maybe a decade or so ago that Proxima Centauri b was detected either by direct imaging or maybe precision radial velocity measurements – the same technique that we used to discover it in 2016.

We can also speculate about the effects of having a whole planetary system a dozen or two AU away instead of the collection of outer planets we have. These worlds would be prime targets for exploration made only more interesting if any of them might support life. Reaching far off Proxima Centauri with any reasonable transit times is beyond our current technology but, it would be a tempting target for early interstellar travel being on a few dozen light days away instead of many light years. In the mean time, we will have to wait an see what planets we find in the α Centauri system so we can get an accurate picture of the skies of our neighboring worlds.

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Related Reading

“Alien Skies: The View from TRAPPIST-1e”, Drew Ex Machina, April 18, 2018 [Post]

“A Sky with Quadruple Suns”, Drew Ex Machina, March 7, 2015 [Post]

Essays about the α Centauri system can be found on the Alpha Centauri page.

General References

Isaac Asimov, “The Planet of the Double Sun”, Fact and Fancy, Discus Books, 1972

Brice-Olivier Demory et al., “Hubble Space Telescope search for the transit of the Earth-mass exoplanet Alpha Centauri Bb”,

Xavier Dumusque et al., “An Earth-mass planet orbiting α Centauri B”, Nature, Vol. 491, pp. 207-211, November 8, 2012

R. Grattonn et al.,” Searching for the near-infrared counterpart of Proxima c using multi-epoch high-contrast SPHERE data at VLT”, Astronomy & Astrophysics, Vol. 638, ID A120, June 2020

P. Kervella, F. Thévenin and C. Lovis, “Proxima’s orbit around Alpha Centauri”,  Astronomy & Astrophysics, Vol. 598, ID L7, February 2017

V. Rajpaul et al., “Ghost in the time series: no planet for Alpha Cen B”, The Monthly Notices of the Royal Astronomical Society – Letters, Vol. 456, No. 1, pp L6-L10, February 2016