According to astronomers at the Harvard-Smithsonian Center for Astrophysics who have been analyzing data from NASA's Kepler space telescope, our Milky Way galaxy could be teeming with planets much like our planet Earth.
The astronomers, in a study that will be published in The Astrophysical Journal (PDF), estimate there may be as many as 4.5 billion Earth-like planets in the Milky Way galaxy alone, with thousands in the neighborhood of our solar system.
According to Space.com, using a sample of 3,897 red dwarfs, the team identified 95 exoplanet candidates circling them. Of the 95, they identified three as being approximately of Earth-size and temperature, orbiting within their stars' "Goldilocks zone," where the planet may retain liquid water needed for life.
The three Earth-size candidates orbiting in the habitable-zone of their parent red dwarf stars were the Kepler Objects of Interest "(KOI) 1422.02, which is 90 percent the size of Earth in a 20-day orbit; KOI 2626.01, 1.4 times the size of Earth in a 38-day orbit; and KOI 854.01, 1.7 times the size of Earth in a 56-day orbit."
According to the astronomers, all three KOIs are located about 300 to 600 light-years from Earth and are in orbit around stars with temperatures between 5,700 and 5,900 degrees Fahrenheit.
The astronomers concluded from the statistics that 6 percent of all red dwarf stars may have an Earth-like planet. Given that red dwarfs are very common in our galaxy, being estimated at three out of four stars, astronomers said we may have billions of Earth-like planets as close neighbors on the intergalactic scale.
Specifically, they determined from an estimate of 100 billion stars in our galaxy that there are 75 billion red dwarfs. Based on the 6 percent estimate they derived an estimate of 4.5 billion "alien Earths" in the Milky Way galaxy. The researchers noted, however, that these figures are tentative.
David A. Aguilar (CfA)
This artist’s conception shows a hypothetical habitable planet with two moons orbiting a red dwarf star. Astronomers have found that 6 percent of all red dwarf stars have an Earth-sized planet in the habitable zone, which is warm enough for liquid water on the planet’s surface. Since red dwarf stars are so common, then statistically the closest Earth-like planet should be only 13 light-years away.
Space.com also notes that according to separate studies conducted by the Research Consortium on Nearby Stars, an international astronomical group led by Georgia State University, within 30 light-years of our sun, there are 248 red dwarfs. The astronomers calculated that if 6 percent of 248 red dwarfs have habitable planets in orbit, then the closest Earth-like world is probably only just 13 light-years away, a distance regarded as being very close on the intergalactic scale, literally a stroll across the park.
When it's young, a red dwarf star frequently erupts with strong ultraviolet flares as shown in this artist's conception. Some have argued that life would be impossible on any planet orbiting in the star's habitable zone as a result. However, the planet's atmosphere could protect the surface, and in fact such stresses could help life to evolve. And when the star ages and settles down, its planet would enjoy billions of years of quiet, steady radiance.
Courtney Dressing, an astronomer at the Harvard-Smithsonian Center, who announced the findings of the research study, said: "We thought we would have to search vast distances to find an Earth-like planet. Now we realize another Earth is probably in our own backyard, waiting to be spotted."
Dressing, presenting the results of the finding (PDF) at a press conference at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., said: "The information we presented today will excite the general public because we now know that the nearest potentially Earth-like world is likely within 13 light years of the Sun. Astronomically speaking, 13 light years is practically next door."
C. Dressing (CfA)
By analyzing publicly available Kepler data, CfA astronomers identified 95 planetary candidates circling red dwarf stars. Of those, three orbit within the habitable zone (marked in green) - the distance at which they should be warm enough to host liquid water on the surface. Those three planetary candidates (marked with blue dots) are 0.9, 1.4, and 1.7 times the size of Earth. In this graph, light received by the planet increases from left to right, and therefore distance to the star decreases f
David Charbonneau, co-author of the study that will be published in The Astrophysical Journal (PDF), said: "We now know the rate of occurrence of habitable planets around the most common stars in our galaxy. That rate implies that it will be significantly easier to search for life beyond the solar system than we previously thought."
Red dwarfs are smaller, cooler and dimmer than our Sun. According to the study authors, an average red dwarf is only about a third as large and only about one-thousandth as bright as the Sun. This explains why no red dwarf is visible to the naked eyes even though the galaxy is teeming with an overwhelming number of them.
Astronomers say, however, that a planet orbiting close to a red dwarf may get sufficient warmth from it to have liquid water that makes it habitable. Red dwarfs are also good places to look for Earth-like planets because the relative size and intensity of signals from the small star and orbiting planet makes it easier to detect the weaker signal of the planet.
While the data available to scientists may not reveal details of what Earth-like planets orbiting dim red dwarfs look like, the Kepler telescope can detect them as objects passing periodically in front of the parent star as in a solar orbital system like ours. Scientists can derive from the data an estimate of the mass and the orbital path of the planet. And as indication of the harvest of Earth-like planets to come in the future, Kepler has already spotted more than 2,700 candidate Earth-like planets in a small area of the sky it has scanned.
The foregoing has significant implications for the question that is never far from human minds: Could there be life on these planets? Could we be living in galaxy teeming with life? The question intrigues scientists because as far as current knowledge goes there are no reasons to suppose that any planet of the right size and temperature that retains liquid water on its surface with organic molecules of life should not evolve living organisms.
As Dressing puts it, "You don't need an Earth clone to have life."