Scientists working with NASA’s Kepler satellite announced last week the confirmation of a planet in the habitable zone around a Sun-like star – Kepler-22b. The habitable zone is the region around a star where water can exist on a rocky planet in its liquid state. This region is also known as the “Goldilocks Zone” because, like the Little Bears’ porridge in the story, the conditions are “just right”. Not too hot, so water boils to steam and the planet has a run-away greenhouse effect, like Venus where the surface temperatures reach the melting point of lead – just outside the inner part of the Sun’s habitable zone. Not too cold, so water on the surface freezes solid, like Mars where any liquid water is trapped underground – just outside the outer part of the Sun’s habitable zone. The Earth sits in the middle of the Sun’s habitable zone with an average surface temperature of 57.2o F, perfect for liquid oceans of water.
The Kepler satellite was launched from Kennedy Space Flight Center in March 2009 to stare at one relatively large patch of the sky along the Milky Way in the constellations of Cygnus and Lyra. The Kepler telescope measures the brightness of over 150,000 main sequence stars, continuously looking for the slight decrease in brightness that would indicate a planet passing in front of its central star – referred to as a transit. These stars are similar to the sun. They are in the long stable stage in which Hydrogen fusing to Helium in the core provides the energy to make the star a luminous sphere of plasma providing the heat and light to their surrounding solar system.
Most solar systems would go undetected by Kepler because the planets do not happen to orbit in an orientation that causes them to pass directly in front of the star. In fact, Kepler would be able to witness the transit of Earth for less than 1% of the solar systems that were identical to ours. However, the large number of stars observed ensure that some fraction will be positioned correctly so that their planets will pass in front of the star. Of course larger planets and planets near the star are most easily detected. Small planets that are the size of Earth are more difficult. The image to the right shows the difference between a large planet like Jupiter and a small planet like Earth transiting the central star of their solar system. To confirm a planet’s discovery repeated transits must be observed. So far, approximately 1,100 planet candidates have been detected.
Now, Kepler scientists have confirmation that a planet was found around a star 600 light-years from us. The planet is almost 2.5 times the diameter of the Earth. We do not know whether or not the planet is a large rocky planet, or a gaseous or liquid planet like Neptune. Answers to those and many other questions will have to wait for the results from further studies. What we do know is that the temperature at that distance from its central star would be around 72oF– nice and comfortable for life as we know it. One artist’s rendition of a possible ocean world is shown to the left.
Less than 20 years have passed since the first planet was confirmed around another star. We learned since then that planetary systems are common around stars. This latest news brings us one step closer to finding planets like Earth, with the conditions favorable for the development of life. We used to look up and only be able to wonder if planets existed. The answers that were once solely in the realm of science fiction speculation, have now been provided thanks to astronomers and instruments like the Kepler satellite. We now see what we could once only just imagine. In the coming years, we will be looking to detect water and oxygen on these newly discovered worlds. What a glorious time in which to live and learn.