Few scientific endeavors have stirred the human imagination as much as the search for exoplanets, those alien worlds orbiting stars beyond our solar system. Since the first such discovery in 1992, the existence of 851 exoplanets has been confirmed, with more than 2,320 likely candidates still in the works.
Perhaps the most intriguing are those exoplanets that lie in their host star’s habitable zone, the range of distance from the star where the temperature is just right for liquid water, and possibly life, to exist. So far, seven such exoplanets have been confirmed orbiting within the habitable zone. However, many are quite close to their host star which means they are likely to be subject to tidal locking, a phenomenon in which gravitational forces cause one side of the planet to always face its star. Tidal locking is also the reason we always see the same side of the moon. For exoplanets, this means that one half of the planet would experience extreme heat from the light of its star while the other half would be frozen in constant darkness. Unfortunately, such a scenario would not be conducive to life.
However, one of the seven confirmed exoplanets in the habitable zone, discovered just last month, is far enough from its star that it has a day and night cycle similar to Earth’s, rendering it more likely to be conducive to life. Its host star is an orange dwarf about 42 light-years from Earth in the constellation Pictor. It is smaller and dimmer than our sun, though not by much. The newly discovered planet, known as HD 40307g, has a mass about seven times that of Earth, and orbits its star every 200 days, as compared to Earth’s year of 365 days. It is the farthest out of the six known planets in its solar system, at a distance of about 56 million miles from its star. In comparison, Earth is 93 million miles from our sun. Though closer to its star, it receives about two-thirds the amount of sunlight that Earth does, making it a bit cooler than our own planet. It is still warm enough, though, to support liquid oceans and wet weather, though the presence of water there has yet to be confirmed. A paper about this discovery will be published shortly in the journal Astronomy and Astrophysics, with Mikko Tuomi of the University of Hertfordshire, UK, as the lead author.
According to researchers at the University of Puerto Rico at Arecibo’s Planetary Habitability Laboratory, the planet Gliese 581g is currently considered the top candidate for hosting life beyond our solar system. A rocky world about 22 light-years away, it is about two or three times the mass of Earth and orbits its star, the red dwarf Gliese 581, every 30 days. This places it in the midst of the habitable zone, just the right place for liquid water, and perhaps life, to exist. Another planet, Gliese 581d, orbiting the same star farther out, is also one of the top candidates on the Arecibo list. Initially thought to be too cold to support life, atmospheric modeling studies now suggest it may be sufficiently warmed by the greenhouse effect.
The more we explore our universe the more likely it seems that life exists out there somewhere beyond the confines of planet Earth. Our own Milky Way galaxy is estimated to contain about 50 billion planets, with about 500 million of them likely to exist within the habitable zone. And the Milky Way is just one of approximately 100 billion galaxies in the observable universe. That makes for a staggering number of places where life, in one form or another, may exist.
While there is no public STARS Club meeting in December, please watch out for next year’s exciting line-up of lectures and events.
Copyright © Amanda Jermyn