February 20, 2022

Late February at 5:50 AM looking southeast

Sunday, February 20, 2022

Our nearest neighbor star, Proxima Centauri, may now have three planets going around it. Well, it’s not a new planet, of course—the planets have been orbiting the red dwarf star for billions of years before humans ever looked up at the sky. It’s just that it hasn’t been until now that we have the capability of detecting it.

The discovery was announced in the current issue of the peer-reviewed journal Astronomy and Astrophysics. However, it is not a confirmed exoplanet. The team led by J.P. Faria notes that it is a “candidate” that needs to be confirmed by other independent teams in the normal peer-review process of science. The initial step of peer-review is the “refereeing” of the paper to make sure the data and analysis are good enough to be published. The next step is other scientists not associated with the discovery group seeing if they can also detect the exoplanet or trying to find errors in the data or logic of the analysis.

The newly-discovered exoplanet candidate, “Proxima Centauri d”, orbits closer to the star than the other two. At a distance of just 3 percent the distance between the sun and Earth, the new exoplanet takes just 5 days 3 hours to orbit the star. Proxima Centauri d can survive that close because Proxima Centauri is a faint red dwarf star that emits just 0.15% of the sun’s energy, so the calculated average surface temperature of the exoplanet would be just 304 K (or 88 deg F) without an atmosphere. It would be hotter with an atmosphere, so the exoplanet’s surface temperature could be slightly less than the boiling point of water. 

Because of its candidate status, the third exoplanet around Proxima Centauri is not listed in the NASA Exoplanet Archive but you can find it in the Extrasolar Planets Encyclopaedia. In fact, the second exoplanet “c” also doesn’t appear in the NASA Exoplanet Archive but exoplanets found by looking at the wobble of a star (the “radial velocity method”) usually take a bit longer to be independently confirmed. 

The star wobble (radial velocity) method uses the fact that as the star gravitationally pulls on the exoplanet, the exoplanet also pulls on the star. The star, being so much more massive than the exoplanet, will have a much smaller motion than the exoplanet. The star will appear to wobble about the star-exoplanet system’s center-of-mass point while the exoplanet will have a much greater motion. Unfortunately, we can’t see the exoplanet directly, so we’re stuck using the star’s tiny motion to infer the existence of the exoplanet and its mass. The radial velocity method gives us a lower bound on its mass if we don’t know how much the orbit is tilted with respect to our line of sight. Proxima Centauri d’s mass is at least 0.26 times Earth’s mass (or about twice Mars’ mass).

Proxima Centauri b was discovered several years ago using the radial velocity technique and it is well-confirmed. It orbits at 5 percent the distance between the sun and Earth and takes 11 days 4 hours to orbit Proxima Centauri. It has a mass of at least 1.27 Earths and its surface temperature would be on cool side at just 234 K (or -38 deg F) without an atmosphere. Atmospheres can significantly warm up a planet. Without an atmosphere, Earth’s surface temperature would be a chilly 254 K (-2 deg F) but the natural greenhouse effect warms us up to an average temperature of 288 K or 59 deg F. Some greenhouse effect is good but too much is not, as seen by our neighbor Venus which has a surface temperature of 737 K (867 deg F) instead of being below the freezing point of water without an atmosphere.

Proxima Centauri c is the outermost exoplanet—exoplanet candidate. Also discovered via the radial velocity technique, its existence has not been solidified enough to make it to the NASA Exoplanet Archive. It orbits at a distance of 1.48 times Earth’s distance from the sun and it takes 1928 days (5.3 years) to orbit the star. Its minimum mass is 5.7 Earth masses and has a calculated average surface temperature of just 39 K (-389 deg F) or about as cold as Pluto. The James Webb Space Telescope will be trained on these worlds to see how dense are their atmospheres and the composition of them (assuming that “c” and “d” turn out to be real bona fide exoplanets). The spectra of their atmospheres will enable us to know whether or not liquid water could exist on the surface and maybe even life has changed the chemistry of the atmosphere like life has done on Earth.

Evening sky watchers still get to revel in the brilliant stars of Orion, Gemini, Taurus, and Canis Major. Early risers/night owls will have a treat on the pre-dawn morning of February 24 seeing a waning crescent moon (one day past third quarter phase) right next to Antares, the red heart of Scorpius. They’ll be rising at about 2 a.m. and will comfortably fit together in the field of view of your binoculars. Three mornings later you’ll see Venus, Mars, waning crescent moon all within a fist’s width at arm’s length of each other. You might also be able to pick out Mercury to the left of the moon just before sunrise.

Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com