Earth From Space
Cameras in space tell stories of life on our planet from a brand new perspective.Read Article
Professor Stephen Hawking thinks the human species will have to populate a new planet within 100 years if it is to survive. In this landmark film, Professor Hawking, engineer and radio astronomy expert Professor Danielle George, and former student of Professor Hawking Christophe Galfard, join forces to find out if, and how, humans can reach for the stars and relocate to different planets.
For further broadcast details and to watch online where available, please visit the BBC programme page for this programme.
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What can be learnt from observing exoplanets?
This animated video explores how scientists have studied distant stars to learn more about exoplanets, the invisible planets that orbit them.
Twenty years ago, astronomers didn’t know for certain whether there were planets in orbit around any stars other than the Sun. All that changed in 1995 with the discovery of a planet orbiting the star 51 Pegasi. Today over 800 exoplanets are known with thousands more candidates having been identified (see www.exoplanet.eu for a complete catalogue).
The reasons for the huge advance are twofold: first, technology has improved to the extent that the tiny influences that a planet has on its parent star may now be measured; and second, advances in computing mean that vast quantities of data may now be automatically scanned to pick out the rare signals that indicate the presence of an exoplanet.
Many people assume that astronomers ‘see’ exoplanets directly through their telescopes, perhaps as a pale blue dot sitting alongside a distant star, possibly even observing shifting cloud patterns on its surface. The reality is even more miraculous: in the vast majority of cases, all that is seen is a single, stationary pin-point of light.
Yet by measuring the brightness of that speck of light, astronomers may see a tiny repeated dimming, as though the star is winking at us, indicating the passage of an exoplanet in front of the star. By measuring Doppler shifts in the star’s spectrum, they may infer the star is wobbling back and forth, as it is tugged by the gravity of an exoplanet.
From the depth of the ‘wink’ and the amplitude of the ‘wobble’, the size and mass of the unseen exoplanet can be determined. Knowing this, its density and surface gravity can be calculated, so giving an indication of its bulk composition.
It’s also easy to determine the length of the exoplanet’s year, from the interval between repeated ‘winks’ and ‘wobbles’. From this, the distance of the exoplanet from the star and its mean temperature may be calculated, and therefore whether or not liquid water can exist on its surface may be determined. All this is from seeing a pin-point of light.
But the extrapolation doesn’t stop there. By carefully plotting the tiny variations in light as the exoplanet orbits the star, and passes behind it, it’s possible in some cases to determine the differences in temperature between the night-side and day-side of the planet and to infer seasonal variations in its cloud cover.
Most remarkably of all, when the exoplanet passes in front of the star, a fraction of the star light reaches us after passing through the exoplanet’s atmosphere. By studying the differences between the in-transit and out-of-transit spectra, it’s possible to infer the composition of the exoplanet’s atmosphere.
Many astronomers are confident that the detection of biomarkers in the spectrum of the atmosphere of an Earth-like exoplanet will happen within the next 20 years, if such life actually exists elsewhere in the Galaxy.
Ultimately, it is measurements of this kind that will tell us whether or not humankind is alone in the Universe. Many astronomers are confident that the detection of biomarkers in the spectrum of the atmosphere of an Earth-like exoplanet will happen within the next 20 years, if such life actually exists elsewhere in the Galaxy. It may only be a green slime that could be scraped off a rock with a finger nail, but it would completely transform humanity’s view of our place in the Universe. And all that is possible merely by observing a pin-point of light through a telescope.
For information about the most successful ground-based transiting exoplanet search, see this article about SuperWASP – the Wide Angle Search for Planets, and for information on the NASA mission which is looking for transiting exoplanets from orbit, see this article about the Kepler satellite:
Almost 300 exoplanets have now been confirmed by the transit method. Most are hot Jupiters, but increasingly a number of Neptune-sized, or super-Earth sized planets, have now been found too. Some of the planets discovered are in the star’s habitable zone – the region of space around the star where the temperature is such that water might exist in liquid form on the surface of a planet that lies there.
Clearly such planets are of great interest as offering the potential for harbouring life. Discovering whether transiting exoplanets in a star’s habitable zone might actually host alien biospheres is (amazingly) potentially within reach of observations.
Will Captain Kirk and Spock ever be able to visit other worlds and walk around in their skin-hugging uniforms, breathing the air? NASA’s Kepler satellite is on the way to answering
One of the best ways to find out about exoplanets is to watch as they ‘transit’ across their host star.
Almost every week there is a news item about an ‘exoplanet’ – a planet outside our Solar System. Astronomers have now found over 500 of these planets orbiting around distant stars. The first exoplanets to be found were very odd compared to our familiar Solar System planets: they are big like Jupiter, but so close to their star they are baked to temperatures in excess of 1000°C. Naturally, most of us are more interested in whether there are planets like our own Earth out there.
One of the best ways to find out about exoplanets is to watch as they ‘transit’ across their host star. In our own Solar System, we can see the transits of Venus and Mercury as they pass between us and the Sun. During a transit of Venus, a Venus-sized portion of the Sun’s disc is obscured. If an exoplanet’s orbit happens to be lined up so the planet passes exactly between us and the host star, a similar event occurs. In the case of Venus and Mercury we can actually see the shape of the Sun and the transiting planet, but for exoplanets all we would see on a photograph, even with the most powerful telescopes, is an unresolved point of light. This is because even the nearest star to the Sun is very, very distant. But we can still detect an exoplanet transit.
Copyright free: NASA
The Kepler satellite is a space telescope designed to precisely measure the brightness of hundreds of thousands stars over and over again. By doing this, it reveals the small drop in brightness which occurs when a small exoplanet blocks a small portion of the light from the host star. In this way, it reveals transiting exoplanets. Kepler has been performing its repetitive measurements for almost two years now: if there were a suitably-oriented Earth-twin orbiting one of the huge numbers of Sun-like stars being measured, Kepler should have seen two transits. For closer-in planets, the orbit is smaller and a circuit around the star is completed more rapidly. Because of this, it takes less time to find the very hot, close-in planets, which is one of the reasons they were found first.
The most exciting announcement from Kepler so far is Kepler-10b , a small rocky planet which orbits a star similar to the Sun every 20 hours. This discovery is the first definitely rocky planet we have found outside our own Solar System, the first exoplanet a spaceship could land on. Kepler-10b would certainly not be a good place for Captain Kirk to take a stroll though: it’s far too hot and probably has no atmosphere at all.
NASA, and the scientists involved in Kepler, are keeping the longer streams of fresher data secret for now, but it is possible that an Earth-like exoplanet might be revealed by their analysis quite soon!
Finding cooler transiting planets takes years. Once Kepler has three years of repetitive measurements, the Kepler scientists will be able to confidently identify exoplanets at similar distances from their host stars as Earth is from the Sun. These planets are likely to have similar temperatures to the Earth, and could possibly support life as we know it. On 1 February 2011, Kepler will make its second public data release. This will triple the amount of data which are publicly available with three months of measurements for 165,000 stars. Transiting exoplanets with orbital periods of a month or less will be revealed.
NASA, and the scientists involved in Kepler, are keeping the longer streams of fresher data secret for now, but it is possible that an Earth-like exoplanet might be revealed by their analysis quite soon! After the four-year mission, the Kepler results will tell us whether Earth-sized planets in Earth-like orbits are common. These statistics will tell us whether it is likely we live in a ‘Star Trek‘ universe, where planets like our own are plentiful.
Kepler will not quite have the final word in this adventure though. The stars Kepler is studying are distant from us, so they appear dim. This will make detailed studies of the exo-Earths Kepler reveals rather limited. The European Space Agency (ESA) has a mission called PLATO under consideration which will find the nearby exo-Earths: by studying a large fraction of the brightest stars in the sky, the PLATO mission should find many of the nearest Earth-twins. These planets will be much easier to study in detail, and will be the first destinations for the real-life Captain Kirks.
What has putting astronauts and robots into space done for us back on Earth? Check out our short comic on weather satellites and predictions.
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