Surprising estimates demonstrate the possibility of the existence of tens of billions of worlds like ours.
You have already known that there are more planets in the Milky Way than stars [See Scientific studies demonstrate that there are 160 billion alien worlds in the Milky Way, and There are more planets than stars in the Milky Way, the studies conclude]. What astronomers have discovered is that those rocky planets, not much larger than Earth, are also common in habitable zones around low-luminosity red stars — a previous survey was not sensitive to this class of exoplanets. An international team has calculated that there are tens of billions of such spheres, often called super-Earths, in our galaxy alone, and probably about a hundred in the immediate proximity to the Sun.
This is the first direct measurement of the frequency of super-Earths around red dwarfs, which represent about 80% of all stars in the Milky Way. This first direct estimate of the number of small planets around red dwarf stars was made with the help of the High Accuracy Radial velocity Planetary Search (HARPS) spectrograph installed on the 3.6-meter telescope at the ESO observatory in La Silla, in Chile.
The HARPS team is looking for exoplanets orbiting the most common types of stars in the Milky Way, red dwarfs – also known as M-type dwarfs, which correspond to the coldest of seven spectral types belonging to a simple classification scheme for their temperature and the appearance of their spectrum. These stars are faintly luminous and small compared to the Sun. However, they are very common and live for a long time, making up 80% of all stars in the Milky Way.
“Our new observations obtained with HARPS show that about 40% of all red dwarfs have a super-Earth that orbits in the habitable zone, that is, where liquid water can exist on the surface of the planet,” said Xavier Bonfils, team leader. “As red dwarfs are very common – there are about 160 billion stars of this type in the Milky Way – we arrive at the surprising result that there are tens of billions of these planets in our galaxy alone,” he added.
The ESO VISTA telescope has made the largest deep-field infrared image of the sky. This new image of a typical region of the sky was taken as part of the UltraVISTA survey and shows more than 200,000 galaxies.
Super-Earths and Gas Giants
The HARPS team analyzed a carefully selected sample of 102 stars of this type that can be observed in the southern sky over a period of six years. Nine super-Earths (planets with masses from one to ten Earth’s masses) have been discovered, including two within the habitable zones of the stars Gliese 581 and Gliese 667 C. Combining all the data, including observations of planetless stars, and looking at the proportion of existing planets that could be discovered, the team was able to find out how common the different types of planets are around red dwarfs. As a result, the occurrence rate of a super-Earth in the habitable zone is 41%, ranging from 28% to 95%.
On the other hand, more massive planets like Jupiter and Saturn – the so-called gas giants – are rarely found around red dwarfs. These giant planets (with a mass of 100 to 1000 Earth’s masses) are predicted to appear in less than 12% of stars of this type.
As there are many red dwarf stars close to the Sun, this new estimate means that there are probably about a hundred super-Earth-like exoplanets in the habitable zones of stars in the vicinity of the Sun less than 30 light-years away.
“The habitable zone around a red dwarf, where the temperature is favorable for liquid water on the planet’s surface, is much closer to the star than Earth is to the Sun,” says team member Stephane Udry. “It is known that red dwarfs are vulnerable to stellar flares which cause the planet bathing in ultraviolet radiation and X-rays, making life unlikely.”
One of the planets discovered in the HARPS survey of red dwarfs is Gliese 667 Cc. It is the second planet discovered in this triple star system and it seems to be near the center of the habitable zone.
Although this planet is more than four times heavier than Earth, it is the most Earth-like “twin” found so far and almost certainly has the conditions necessary for liquid water to exist on its surface. It is the second super-Earth discovered within the habitable zone of a red dwarf during this HARPS survey, after Gliese 581d, announced in 2007 and confirmed in 2009.
“Nowadays when we know there are many super-Earths orbiting red dwarfs close to us, we need to identify more of them using both HARPS and future instruments. Some of these planets are expected to pass in front of their host stars as they orbit them giving us an excellent opportunity to study the planet’s atmosphere and find signs of life,” concludes Xavier Delfosse, another member of the team.
The HARPS instrument measures the radial velocity of stars with extraordinary precision. A planet orbiting a star causes it to move back and forth relative to a distant observer on Earth. Due to the Doppler Effect, this variation in radial velocity causes a shift in the star’s spectrum towards longer wavelengths as the star moves away (called redshift) and towards shorter wavelengths as it approaches (blueshift). This tiny shift in a star’s spectrum can be measured with a high-precision spectrograph like HARPS and used to determine the presence of a planet.