According to NASA’s Exoplanet
web page (exoplanetarchive.ipac.caltech.edu/index.html), we know of 3,779 planets
orbiting stars other than our sun, with 2,737 more candidates awaiting
confirmation. And that is just the tip of the exoplanet iceberg.
The large majority of confirmed
and suspected exoplanets are discovered by the transit method. A telescope in
space or on Earth stares at a star and watches for small drops in the light
output that indicate a planet is passing in front of, or transiting, the star.
There are many ways a star’s light may vary, but each has a specific signature
as to how the brightness varies. Planetary transits cause a unique alteration
in the star’s light.
This method can only detect
planets whose orbit lies along our line of sight, and that’s quite unlikely.
That astronomers have found so many exoplanets when they can only detect such a
tiny fraction of potential candidates implies a huge number of exoplanets
exist. In fact, astronomers estimate based on the known sample that the 400
billion stars in the Milky Way average 1.3 planets each.
That’s a lot of planets.
It turns that that one
exoplanet is literally right next door. The closest star system to us is Alpha
Centauri. It actually consists of three stars. Two of them, Alpha Centauri A
and B, both roughly the size of our sun, orbit each other rather closely. The
third member, named Proxima Centauri, orbits those two in a wide, 550,000-year
orbit. Proxima comes closer to us than any other star, 4.2 light years at its
closest.
Proxima is known to possess a
planet only slightly larger than earth. And the planet lies in the star’s
habitable zone, where the star provides enough heat to allow liquid water, as
on Earth. Since Proxima is a red dwarf star, much smaller than our sun, the
planet must orbit close to the star to be warm enough. The planet’s orbit takes
only 11 days. But it is exactly in the middle of Proxima’s habitable zone.
Being so close to the parent star, it is probably tidally locked. One side
constantly faces the star, just like only one side of the Moon always faces Earth.
This means one side is in constant daylight, the other perpetual night.
Artist conception of Proxima Centauri b - credit NASA
Anthony Del Genio, a planetary
scientist at the NASA Goddard Institute for Space Studies, led a group of
scientists doing computer simulations on Proxima b. They wanted to know if the
planet could support life. They made the reasonable assumptions that the planet
had an atmosphere as thick as Earth’s and enough water to form an ocean. Using
computer models like those used by researches to study climate change on Earth,
they found that under a broad range of conditions, the planet can sustain liquid
water even on the night side. On Earth, where there’s water, there’s life. “The
major message from our simulations is that there’s a decent chance that the
planet would be habitable,” said Del Genio.
Our nearest habitable neighbor
may literally orbit our nearest stellar neighbor.
On the first Tuesday of each month, I write an
astronomy-related column piece for the Oklahoman newspaper. On the following
day, I post that same column to my blog page.
