Are we alone in the universe?
The existence of Kepler-22b, the first earth-like planet in the habitable zone of a distant star, is a shot in the arm for scientists looking for signs of life elsewhere, writes G V Joshi
It was in December 2011 that astronomers working at NASA’s Kepler mission in th US confirmed the existence of the first earth-like planet in the ‘habitable zone’ of a distant sun-like star. The newly confirmed planet, Kepler-22b, is about 2.4 times the radius of earth.
Scientists don’t yet know if Kepler-22b has a predominantly rocky, gaseous or liquid composition, but its discovery is a step closer to finding earth-like planets.
In astronomy and astrobiology, the habitable zone is the region around a star where a planet of earth-like size, composition and atmospheric pressure can maintain liquid water on its surface. In other words, it refers to the belt of orbits around sun-like star where liquid water could exist on a planet’s surface. In the words of Dr Douglas Hudgins, Kepler programme scientist at NASA Headquarters in Washington, USA,“This is a major milestone on the road to finding Earth’s twin."
Kepler-22b is located 600 light-years away from earth. In other words, what we are seeing today has happened 600 years earlier. While the planet is larger than the earth, its orbit of 290 days around a sun-like star resembles that of our world. The planet’s host star belongs to the same class as our sun, although it is slightly smaller and cooler. It is a bit smaller than our sun, so its habitable zone is slightly closer in.
Kepler 22b, as it is known, is 2.4 times the size of the earth and takes 290 days to orbit its star. If it had a reasonable atmosphere, the surface temperature on Kepler 22b would be about 220 to 230 C, “a very pleasant temperature.” But whether Kepler 22b is actually habitable depends on its composition and atmosphere, neither of which is known at present.
National Aeronautics and Space administration’s (NASA) spacecraft carrying the Kepler Telescope (KT) lifted off on March 6, 2009 aboard a Delta II rocket from Cape Canaveral Air Force Station in Florida.
It is named after Johannes Kepler, the 17th century German astronomer who discovered the three laws of planetary motion and confirmed that it is the earth that revolves round the sun and not otherwise, as was thought in the West for nearly 16 centuries. The spacecraft carrying KT is specifically designed to survey the region of the Milky Way galaxy to discover hundreds of earth-size and smaller planets with the potential to host life and study the hundreds of billions of stars in our galaxy that might have such planets.
The spacecraft has been launched into an orbit where it will drift behind the earth as it circles the sun. This will ensure that the earth does not block the light from the stars which need to be observed continuously by the observatory.
Engineers acquired the first signal from KT immediately, after it separated from its spent third-stage rocket and entered its final sun-centered orbit, trailing 1,600 km miles behind earth.
The KT will orbit the sun to watch a patch of space thought to contain about 100,000 stars like our sun continuously for four to over six years.
It is the first mission designed to find rocky earth-like planets orbiting sun-like stars.
It will look for the slight dimming of light from these “suns” as earth-like plants pass between them and the spacecraft. KT has the largest camera system ever launched into space.
The KT can observe and record changes in brightness as low as 20 parts per million. If KT were to look down at a small town on earth at night from space, it would be able to detect the dimming of a light as somebody passed in front of it.
Why are astronomers looking for earth-like planets?
Earth is unique in the solar system as being the only planet which is able to support life as we know it in all its forms: from basic living micro-organisms to highly sophisticated and intelligent human beings. There are many reasons for this.
Earth has a breathable atmosphere containing 78 per cent nitrogen and about 21per cent oxygen the gas that is required for the life of most creatures. Oxygen is constantly put into the atmosphere by plants and trees by a process called photosynthesis. The remaining one per cent consists chiefly of argon, with only extremely small amounts of the other inert gases like Helium. It also contains water vapour.
Earth’s atmosphere does not contain much carbon dioxide, a poisonous gas which makes up most of the atmosphere of planets like Venus and Mars. The earth's atmosphere is kept on the planet by its pull of gravity.
Earth has a habitable climate
Earth has water. Water is believed to be the most important chemical necessary for life.
All of the reasons given above for life existing on earth are only possible because of one main reason, the eun, about which it revolves in about 365 .25 days. Because of earth’s ideal distance from the sun, it receives the perfect amount of heat and light to allow life to be created and to support it. Put simply, if there were no sun, there would be no life on earth.
In addition the earth has a moon. Our moon has played a subtle but necessary role in the evolution of life here on earth. The moon exerts strong tidal forces, and stabilises the tilt of earth’s rotational axis (between 22.1 and 24.5 degrees) to keep the seasons constant.
At the same time, the earth has the ideal size in terms of magnitude to possess an atmosphere. If the mass of the earth were a little less, then its gravitational force would be insufficient and the atmosphere would be dispersed into space. If its mass were greater, then the gravitational force would be too much and humans would not be able to walk on their two legs.
Another crucial balance for human life is the ozone level in the atmosphere. If it were greater than its current value, the surface temperatures would be too low. If it were less, surface temperatures would be too high, and there would be too much ultraviolet radiation at the surface.
Astronomers, therefore think that if there were life as we know it anywhere in the universe, it could be on a planet like earth, revolving about a star like our sun. In the words of Dr William Borucki, the science principal investigator for the Kepler mission at NASA’s Ames Research Center in Moffett Field, California, “Even if we find no planets like earth, that by itself would be profound. It would indicate that we are probably alone in the galaxy.”
In the words of Dr Edward Weiler, Associate Administrator for the Science Mission Directorate at NASA, “This is a historical mission; it’s not just a science mission. It really attacks some very basic human questions that have been part of our history, since that first man or woman looked up into the sky and asked the question: ‘Are we alone?'”