nd in the distribution of exoplanet masses, with smaller planets outnumbering larger ones. Extrapolating from their data, the research team estimated that 23 percent of stars in our galaxy, or 46 billion stars, would have earth-sized planets orbiting the hot zone close to the star.
This result contradicts current theories on planet formation, which suggest that smaller planets would tend not to form in closer orbits. Nevertheless it gives support to the existence of large numbers of earth-size planets in more distant orbits and so possibly habitable. Andrew Howard of the University of California, Berkeley, lead author of the study explained, “During planet formation, small bodies similar to asteroids and comets stick together, eventually growing to Earth-size and beyond. Not all of the planets grow large enough to become giant planets like Saturn and Jupiter.” Howard added, “It’s natural for lots of these building blocks, the small planets, to be left over in this process.”
A series of discoveries of exoplanets with fascinating properties have been made of recent months. In September researchers from the University of California, Santa Cruz, and the Carnegie Institution of Washington, announced they had found two exoplanets orbiting around the nearby red dwarf star Gliese 581. Their findings are based on 11 years of observations at the Keck Observatory. One of these planets, Gliese 581g, has a mass three to four times that of the Earth and an orbital period of fewer than 37 days. It is thought to be the first exoplanet discovered that lies in the habitable zone of the star, with temperatures in the range required for liquid water to exist on the planet’s surface and to sustain an atmosphere.
In November, the first exoplanet from outside of our Milky Way galaxy was found. This Jupiter-sized planet orbits a star known as HIP 13044 that is part of the so-called Helmi stream. These are stars that originally belonged to a dwarf galaxy that was “eaten up” by our Milky Way some six to nine billion years ago.
Last month an exoplanet was discovered for the first time with an atmosphere that could be measured. The planet, GJ 1214b, is three times larger than earth and with a mass seven times greater. This study used near-infrared observations at the Very Large Telescope at Paranal Observatory in Chile. Although the planet has been shown to have an atmosphere, the chemical composition is still to be determined, with hydrogen, helium or water-steam all being possibilities.
Also in December a team of US and British scientists, using observations from NASA’s Spitzer Space Telescope, discovered exoplanet Wasp 12b. This Jupiter-sized gas planet has been found to contain a high proportion of carbon, suggesting that rocky exoplanets made of diamond or graphite could exist.
Several observations over the last two years have produced visual images of exoplanets. The first such image was obtained in 2008, of three exoplanets orbiting a bright star called HR 8799, lying some 129 light years from Earth and faintly visible to the naked eye. Recently further analysis on HR 8799 has shown up a fourth exoplanet (see picture). Together with the fact that there are two belts of debris orbiting the star—small rocky or icy objects, as well as large amounts of tiny dust particles – the existence of four giant planets makes HR 8799 similar to our own solar system though on a much larger scale.
Our solar system has four giant planets—Jupiter, Saturn, Uranus and Neptune, and our debris belts include the asteroid belt between the orbits of Mars and Jupiter and the Kuiper Belt, beyond the orbit of Neptune. Christian Marois, an NRC astronomer and lead author of the research which is published in Nature  said, “The images of this new inner planet are the culmination of 10 years’ worth of innovation, making steady progress to optimize every aspect of observation and analysis. This allows us to detect planets located ever closer to their stars and ever further from our own solar system.”
 Science, Vol. 330, 29 October 2010, p. 556.
 Nature, Vol. 468, 23 December