Planet Disovered Just Next Door


Raşit Gürdilek

Can life exist under a red haze?

Following weeks of rumors, official word was finally out:  dwarf star Proxima Centauri, just 4.2 light years away, has an Earth-like planet orbiting within the star’s “habitable zone.”

The planet, discovered through dedicated observations with telescopes at the European Southern Observatory (ESO) in Chile and subsequently confirmed by further observations with a constellation of other telescopes under a collaborative effort, was named Proxima b. The first signs of a possible planet orbiting the Sun’s closest neighbor were detected in 2013 but could not be confirmed. Through continuous observations restarted in the first half of this year, the planet was nabbed from the wobbles it caused to the star as they orbited a common center of gravity (within the star) causing it to periodically move toward the Earth at five kilometers an hour (normal walking speed of a human), which were detected by extremely sensitive instruments attached to the telescopes.   

Since the star is far less massive than the sun and hence exerts a weaker gravitational pull, the planet orbits it from very close. Its orbital distance to the star is about 5 percent of an Astronomical Unit (AU) which is a yardstick corresponding to the average distance between the Earth and the sun which is 150 million kilometers.  Hence its orbital period (year) is just 11.2 days.

Since the energy Proxima b receives from its star amounts to 65 percent of what the Earth gets from the Sun, Astronomers do not categorically rule out the existence of conditions suitable for life on the planet despite its proximity to the star.

Findings of the study carried out by a team composed of astronomers from Belgium, France, Germany, Spain, U.S. and U.K. and published in Nature, show that because of its tight orbit, Proxima b stays within a disk-shaped swathe of space called the “habitable zone” where temperatures permit the existence of liquid water essential for life as we know it.  

Still, long-duration new observations are required to determine whether it indeed is hospitable to life. But certain characteristics of the red dwarf stars and the proximity of the habitable zones to these low-mass stars erect some obstacles to the emergence of life on planets within their habitable zones, the presence of liquid water notwithstanding. 

 One of these is the hyperactivity of these stars in their youths, causing them to emit lethal radiation and eject charged particles during magnetic upheavals called flares which are much stronger than those seen on the sun.

Another is the planets getting caught in what is called the gravitational trap because of their closeness to the stars, with the result that the time it takes for them to revolve once around their axes of rotation – or their day – corresponds to the time it takes them to complete an orbit around their stars – their year.  Thus one hemisphere constantly faces the star and gets heated to extreme temperatures whereas the other hemisphere freezes in eternal darkness.

The energy Proxima b receives from its star is about two-thirds of the amount the Earth gets from the sun. Some 70 percent of up to 400 billion stars estimated to reside in the Milky Way galaxy is thought to be made up of red dwarf s like Proxima Centauri (see: KURIOUS) Their masses can be as low as a tenth of the sun and their lifespans can be as long as 10 trillion  years.  As stars from this spectral class emit most of their energy in the infrared wavelengths, the skies on their planets are speculated to display pinkish-red hues (see the artist’s conception at top). But while the infrared and visible light provide heat to the planet, intense X-ray and ultraviolet radiation they emit may sweep atmospheres of nearby planets into space and boil away any water on their surfaces.

Proxima b is exposed to this intense radiation 60 percent more compared to similar radiation that falls on Earth from the sun, and thus could have lost the bulk of its atmosphere and surface water over its 5 billion year past. Although it is not known how much water the planet initially had on its surface, astronomers estimate that it could have lost an ocean’s volume of water within its first 100-to-200 million years.

So, the newly discovered planet may be a dry world devoid of an atmosphere, or could have somehow held on to its atmosphere and most of its water.

Proxima Centauri ,credited now with at least one planet, is the minor component  of  a triple star system simply called Alpha Centauri in the constellation Centaurus, visible from the Earth’s southern hemisphere as one of the brightest stars in the sky. The system is composed of Alpha Centauri A, which is a “yellow” star of the spectral class G like the sun although slightly more massive and hotter, Alpha Centauri B, which is an orange, K-class star slightly less massive and cooler than the sun, and the Proxima Centauri, an M-class red dwarf, one of the least massive but most populous stars in the Milky Way, thought to be orbiting its larger two companions. But the assumption that Proxima Centauri is the third member of the triad requires proof , since while the larger two members orbit each other from just 16 billion kilometers, Proxima Centauri  lies a quarter of a light year away (1 light year  is about 9.5 trillion kilometers) and  provided it’s  a member , it has to complete an orbit in 500,000 years. Because of the length of the orbital trajectory, astronomers shirk from a definitive verdict as to its status.

REFERENCES

  • 1. “Planet found in habitable zone around nearest star”, ESO, 24 August 2016 “Rocky planet found orbiting habitable zone of nearest star”, Carnegie Institution for Science, 24 Ağustos 2016
  • 2. “Earth-like planet found orbiting our nearest stellar neighbor”, ScienceOnline, 24 August 2016 The habitability of Proxima Centauri b
  • 3. http://www.ice.cat/personal/iribas/Proxima_b/index.html