What is Alpha Centauri's 3rd Planet?
The planetary family around Proxima Centauri, the closest star to Earth, just keeps growing.
Let’s be clear, Alpha Centauri is the closest solar system to Earth’s. A little over four light-years away is Alpha Centauri, the closest star system to the Sun.
And in August 2016, researchers announced they’d found a long-searched-for planet around Proxima Centauri, the smallest — and closest — component of this three-star system.
Then, in January 2020, astronomers spotted a second world around Proxima Centauri. And now, the star’s family tree appears to be growing again: A third terrestrial planet has been found orbiting the nearest star to Earth.
Proxima d might be only a quarter the mass of Earth, potentially making it one of the smallest worlds yet discovered beyond our solar system.
In a study published Feb. 10 in Astronomy & Astrophysics, astronomers announced the discovery of Proxima d. This tiny planet, weighing in at just one-quarter the mass of Earth, orbits Proxima Centauri every five days at a distance of some 2.5 million miles (4 million kilometers), or less than one-tenth the distance of Mercury from our own Sun.
“The discovery shows that our closest stellar neighbor seems to be packed with interesting new worlds, within reach of further study and future exploration,” study lead author João Faria, a researcher at the Instituto de Astrofísica e Ciências do Espaço in Portugal, said in a statement.
While today 4.7 light years seems like an unimaginable distance from us in 2022, that might not be the case twenty or fifty years from now. Alpha Centauri is a gravitationally bound system of the closest stars and exoplanets to our Solar System at 4.37 light-years from the Sun.
Why Red Stars can Be Exciting for the Search for Life
Proxima Centauri is a red dwarf with only about 12 percent the Sun’s mass and 14 percent its diameter, this puts Proxima d in the star’s habitable zone, where conditions are just right for liquid water to potentially exist on its surface.
Proxima d was discovered using the radial velocity method, during which astronomers carefully watch a star to look for subtle changes in position, which occur as the gravity of an orbiting planet tugs on its star. This is the same method used to detect Proxima Centauri’s other two planets, Proxima b and c. But these planets are more massive — in fact, Proxima d is the lightest exoplanet to date ever discovered using this method.
There are at least three planets then just right next to us as stellar neighbours.
Proxima b, the first planet discovered around this star, is roughly the same mass as Earth and orbits every 11.2 days at a distance of 4.3 million miles (7 million km), or about 5 percent the distance Earth orbits the Sun. But again, because Proxima Centauri is much smaller than our star, Proxima b also orbits in its habitable zone.
Proxima c is different: It’s a super-Earth weighing in at about six times our planet’s mass, with an orbital period of 5.2 years. That puts its orbit, at about 130 million miles (224 million km), beyond the habitable zone.
Researchers discovered Proxima d using a new instrument called the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations, or ESPRESSO, on the European Southern Observatory’s Very Large Telescope in Chile. Now that the instrument has shown its mettle, astronomers are hoping to use it to uncover many more terrestrial worlds perhaps much like our own, both close to and far from home.
In recent years exoplanets have become quite common and we now know our own galaxy is likely full of life, though the timing between intelligence sentiment life (thriving and then going extinct) that are spare-faring is still likely rather rare.
A Galaxy Full of Rocky Planets in the Habitable Zone
This discovery is also rather recent and represents the 3rd planet found at Alpha Centauri, where we once believed there were none. . On February 10, 2022, researchers using the European Southern Observatory’s Very Large Telescope (VLT) said they found a third planet next door. This suggests that rocky planets may be more common than we thought even ten or twenty years ago.
In 2020, on average, astronomers estimate that each sunlike star in the Milky Way likely harbors between 0.4 and 0.9 rocky planets in its "habitable zone," the just-right range of orbital distances where liquid water could be stable on a world's surface, researchers have found.
Over time I’m not sure this artificial construct of the Habitable zone will even matter so much. About 7% of the Milky Way's 200 billion or so stars are "G dwarfs" like the sun, so that's a lot of possibly Earth-like real estate. Scientists define "rocky planets" as worlds with diameters 0.5 to 1.5 times that of Earth, and sunlike stars as those with surface temperatures between 8,180 and 10,880 degrees Fahrenheit (4,527 to 6,027 degrees Celsius).
The habitable zone (HZ) is a decidedly squishy concept; whether a planet resides in it depends on the thickness and composition of its atmosphere and the activity level of its host star, among other factors. Scientists on Earth seem to be very enamored with water being necessary for life. The habitable zone is tailored to water-dependent Earth-like life, and it doesn't consider subsurface liquid water, which can exist on very cold, airless worlds, as some of the moons of Jupiter and Saturn show. However the true diversity of life in the actual galaxy and universe might be more diverse.
A 2013 study based on Kepler data estimated that about 6% of red-dwarf systems boast a roughly Earth-like planet in the habitable zone, and one such world is the closest alien world to our solar system, at a distance of merely 4.2 light-years — Proxima b, which orbits the red dwarf Proxima Centauri. As our knowledge increases this number is actually likely to be much higher.
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