An artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. Image Credit: ESO/M. Kornmesser
The nearest star to Earth (aside from the Sun) might have an Earth of its own, scientists announced today. Proxima b, which orbits the star Proxima Centauri, is a rocky world about our size, mass, and temperature. It orbits within what astronomers call the "habitable zone," and it is able to harbor liquid water on its surface.
In other words, it might be able to support life.
The findings, published today, August 24, in Nature, are the result of work by a research team of more than 30 scientists from around the world who have been documenting their search for just such a planet through the Pale Red Dot campaign (in honor of both Proxima's assumed red hue and the Pale Blue Dot we call home).
For all the similarities, there are some significant differences between Proxima b and Earth. One year on Proxima b would be 11 Earth days long, and its distance from its host star is less than that of Mercury to the Sun. (Despite being the planet nearest to the Sun, Mercury isn't the boiling cauldron you might expect. It's not even the hottest planet in the solar system—that honor belongs to Venus, whose surface would melt solid lead. Mercury, on the other hand, possesses water ice.) Proxima Centauri is much cooler and smaller than our Sun—it's closer in size to Jupiter—leaving the rocky world of Proxima b suitable for water, atmosphere, and life.
An exoplanet is a world that orbits a star other than our own. (Scientists have also identified "rogue" exoplanets, which orbit no star at all.) There are more than 3500 identified exoplanets in the universe, orbiting thousands of different stars. Thousands of additional candidate exoplanets have been spotted and await further study before being made official.
Such planets are discovered using several methods. When a planet's orbit places it between its star and Earth, the amount of light we receive from the star is diminished. Dimming over a consistent interval might suggest a planet. Conversely, a planet can sometimes make a star seem brighter, depending on its closeness to the star and the thermal radiation it thus emits. Planets with significant mass have a gravitational pull on stars. We can detect such movements by way of the Doppler effect. Electromagnetic waves produced by a star as it approaches the Earth are of a different frequency than waves produced by a star as it moves away. Shifting frequencies of a star suggest a wobble, and therefore the possible influence of a planet.
It is the latter technique scientists used to find Proxima b. An observed wobble doesn't automatically mean an exoplanet; sunspots can have the same apparent effect. However, research by John Barnes of the Open University in England ruled out sunspots as the cause of Proxima Centauri's oscillations. "Once we had established that the wobble wasn’t caused by star spots, we knew that there must be a planet orbiting within a zone where water could exist, which is really exciting," he said in a press statement.
SECOND STAR TO THE RIGHT
Proxima Centauri is a red dwarf that's much smaller than our Sun—only about one-eighth the mass—and cannot be seen by the naked eye. It has a lower luminosity, though does exhibit occasional, intense flares of brightness. Such activity, combined with other factors, means that the star has a good four trillion years of life left in it. (The universe itself is only 14 billion years old.)
Before you start packing for the journey to visit our neighbor, know that Proxima Centauri is approximately 38.25 trillion kilometers, or about four light-years, from Earth, and it would take us tens of thousands of years to reach it with existing propulsion systems. Still, studying the star and its newly discovered rocky world will be made much easier once the James Webb Space Telescope, the successor to Hubble, launches in 2018 and the European Extremely Large Telescope comes online in Chile in 2024. If subsequent discoveries indicate the presence of a friendly atmosphere, it would be, according to Barnes, "arguably one of the most important scientific discoveries we will ever make."