Planet hunting is a very new and exciting field of astronomy. In fact, the first planet outside our solar system was not discovered until the mid-Nineties, as the telescopes and methods needed to find planets were simply not advanced enough before then. Today there are thousands of known planetary candidates and potentially trillions more In our Milky Way alone. So far we’ve found gas giants larger than Jupiter, Earth-like planets more than twice as big as our world and even some rocky bodies smaller than our home planet. With each new discovery we learn a little bit more, not only about worlds outside our solar system (which are known as exoplanets or extrasolar planets), but also those inside it, including Earth.
However, as new as planet hunting may be, there’s no doubt that the ultimate goal Is to answer a question that has perplexed humanity for hundreds, or perhaps even thousands, of years: are we alone In the universe?
Statistically speaking, the chances of our planet playing host to the only life forms in the cosmos are incredibly small. Consider, first, our own solar system. In It are eight planets and five subsequent dwarf planets, although of these only Earth is known to harbour life. Some of these planets and their moons appear to have once had the capability to support life, such as Mars, or perhaps still do today, such as Jupiter’s moon Europa, but so far we have found no concrete evidence that they do.
Missions such as NASA’s Mars Science Laboratory, due to land on the Red Planet In August 2012, will give us a greater understanding of the ability of life to adapt to different environments, but even the most optimistic estimates don’t expect us to find anything other than small microbial life in the solar system, and certainly not intelligent life forms.
So for our true walking, talking extraterrestrials we need to head out of the solar system. A study in early-2012 suggested that every star plays host to an average 1.6 planets, although some stars are too large or volatile to be thought to have orbiting exoplanets. However, upper estimates for the number of stars in the Milky Way reach a figure of 400 billion, so If even just one star in every 400 has a planet, that’s still a potential one billion planets in our own galaxy. The mind truly boggles when you then consider that the universe is thought to comprise up to 200 billion galaxies, bumping our upper estimate of planets to 200 million trillion. If these figures turn out to be accurate, we’ve so far discovered much less than a millionth of all the planets in our galaxy and an even more incredibly small fraction of the planets in the universe.
Could it really be that out of all of these only one – Earth – had the necessary conditions for life to develop? Does only one of these orbit its parent star at a habitable distance? Is Earth really the only planet with water, vegetation, a temperate climate and a breathable atmosphere? It would take a hardened sceptic to unequivocally rule out even the slightest possibility of life elsewhere, and we’re certainly not one of them. And neither are the thousands of scientists around the globe looking for planets that fit the criteria of being ‘Earth-like’ with the hope – and conviction – that one day we’ll discover a planet that ticks all the boxes. We could then send a signal in anticipation of a response or, much further down the line, launch an Interplanetary spaceship to explore this strange (or perhaps familiar) new world.
The hunt for ET life
Of course, we haven’t taken into account other factors for life on a planet. It took roughly 4.7 billion years for Intelligent life to flourish on Earth, and the constant threat of nuclear war, asteroid collisions and other disasters means that the window during which intelligent life is active is relatively small. Indeed, this might help explain a perplexing paradox: if intelligent life is so abundant in the universe, why has no one ever got in touch with us? This very paradox was first postulated by physicist Enrico Fermi in the Forties, when he said that the lack of extraterrestrial contact with our planet suggested that interstellar travel was impossible, but in truth we’re no closer to the answer than he was then. However, there have been several efforts to disprove Fermi, which we’ll touch on later.
On Earth we are increasing our capability to hunt for new planets with ever-bigger and more powerful telescopes, while our world is constantly emitting signals – both accidental and deliberate – from our multitude of electronics. If someone or something else is out there, wouldn’t they have found us by now? Maybe we haven’t been actively sending signals out long enough to reach a nearby planetary system, maybe we’re being ignored, or maybe we truly are alone. The best way of answering these questions is through the search for planets outside our solar system, which is why it has become such an important area of astronomy despite the fact it’s barely three decades old.
The problem for a long time has been finding planets that are similar to Earth. At first, all we could find were large ‘hot Jupiters’, which are gas giant planets that orbit their star at a very close range In a very short period of time and thus are extremely hot Our current methods of finding planets are limited and we have to rely on indirect methods – mainly observing the effect a planet has on a nearby star to detect an exoplanet. It was not until the arrival of NASA’s Kepler telescope In 2009 that smaller planets were able to be spotted, as the use of the transit method proved largely successful. In the future, as bigger and better telescopes are built, it is likely we will be able to directly photograph exoplanets. This would be a massive leap for planet hunters, as a whole host of new data could be gleaned from planets, plus ever-smaller ones would be detectable.
Only in the past year have we truly started to make ground in finding Earth-sized planets. In fact, the first planet smaller than Earth was not discovered until 20 December 2011. This was Kepler-20e, a rocky planet with a radius 0.87 times that of Earth. Although it orbits too close to its host star to have liquid water on its surface, its discovery was one of the most important made thus far in the brief history of planet hunting. While we’d found gas giants and super-Earths – so-called giant terrestrial planets – we had yet to discover a planet similar in size to our own. Its discovery indicated that there could well be Earth-sized planets orbiting stars at a safe distance, much like Earth, and with billions, or even trillions, of planets in the Milky Way alone, it’s not unlikely that we’ll find such a planet In the near future.
Along with the discovery of familiar planets to those in our own solar system have been bizarre new worlds that have changed our understanding of planetary science. Of particular interest are super-Earths – exoplanets with a mass greater than Earth’s but a composition that appears rocky. It was previously thought the larger a planet Is the more likely It Is to be a gas giant, but now it Is generally agreed that a rocky terrestrial planet can be up to ten times the mass of Earth. What actual conditions on these super-Earths might be like, though, are still – for the most part – up for debate.
The first super-Earths to be discovered In a habitable zone were two planets in orbit around the star Gliese 581. The significance of being within the habitable zone Is that this is the distance from the host star at which it is calculated that a planet can support liquid water, one of the known components needed for life to start or flourish. Designated ‘c’ and ‘d’, the former planet has a mass of about five Earths and the latter 7.7. While Gliese 581 c is probably too hot to support ET life, d sits squarely In its host star’s habitable zone. However, due to its mass it is estimated to have a very strong gravitational pull, and its orbit of just 0.22 AU from its host star shows that even a planet within a habitable zone may not have all the prerequisites for life.
Apart from super-Earths there are countless other types of exoplanet that we are only just now beginning to understand. PSR J1719-1438 b, for instance, is a pulsar planet that appears to be so dense that it is thought to be made of diamond. Other planets orbit their star at such a close distance, or at such a high speed, that the surface conditions might be unlike anything we could imagine. As we discover more and more planets similar In size and composition to our own, the prospect of finding one that ticks all the boxes for being habitable grows increasingly likely. As it is such a new area of astronomy, the next few years will be a period of great anticipation for planet hunting. We won’t be visiting one of our neighbours for the foreseeable future, but we can do our best to determine if other planets possess the ability to support life, or maybe even find evidence of life by finding artificial signals emitted like those from Earth, or breathable atmospheres that indicate the presence of life forms. With each new planet that is confirmed we take one step closer to determining if we are indeed alone In the universe, or one of many.
For a planet to have the potential to support life, it must lie within a region known as the habitable, or ‘Goldilocks’, zone, around its host star. If it’s too close It will be scorchingly hot, but too far away and it won’t receive enough light and heat to support life. In addition, planets beyond the habitable zone of a star tend to be gas giants, as during the formation of a planetary system the colder, gaseous planets can only form farther out, while the rocky terrestrial planets form closer to the star. Our solar system Is a prime example; beyond Mars, which is said to be at the edge of our habitable zone, you’ve got Jupiter, Saturn, Uranus and Neptune – all gas giants. However, between Mars and the Sun are Earth, Venus and Mercury. All are rocky terrestrial planets, although only one – Earth – is at the necessary distance for water, and so life, to exist.
The key to finding a liveable exoplanet Is to find one located In the habitable zone of its host star. Due to the primitive methods of finding planets currently at our disposal, the majority of planets found so far have been large, hot gas giants orbiting very close to their host star. It Is only recently that we have begun to find exoplanets of a similar size to Earth, while water-bearing planets have been much scarcer. The farther a planet is from its host star, the harder It is to find. As mentioned earlier, it’s hoped that, eventually, when we are able to directly Image exoplanets, Earth-like ones will appear more regularly.
Planet Hunting Methods
There’s a reason why the first exoplanets weren’t discovered until the Nineties: finding them is incredibly difficult. While seeing stars with the naked eye in the night sky is easy, observing planets is much harder. Planets don’t give out their own light, instead only reflecting that of nearby stars, and thus are far less luminous. As a result scientists needed to devise alternative methods to spot planets…
This is the predominant method through which the majority of planets have been found. When a planet crosses in front of a star it causes a momentary dip in brightness. The amount the star dims is relative to the size of both the star and the planet; the size and mass of the star in question can be measured using a spectrometer. Although the planet cannot be directly observed, its mass and size can be estimated by measuring the star’s fluctuation In luminosity.
For a planet orbiting a Sun-like star at 1AU (the distance from Earth to the Sun), the probability that its orbit will transit in an observable manner in front of the star is about 0.47 per cent. Thus it can take more than 200 observations of one star to definitively confirm or deny the presence of a planet, and even then the speed of the planet’s orbit could lower the probability further. The transit method also suffers from a high rate of false detections, and so a follow-up detection method such as the radial-velocity method is needed to confirm or refute the findings.
Radial-velocity measurement, aka Doppler spectroscopy, observes the Doppler shifts in the spectrum of a distant star. To understand how it works, Imagine the Sun-Earth system. We know Earth orbits the Sun, but the Sun actually orbits the Earth as well. For this reason, In the Sun-Earth system, there is a central point around which both objects orbit. However, as the Sun is so much larger, this point Is much closer to the centre of the Sun; in fact, the point is only 450 kilometres (280 miles) from the centre of our solar system’s star. Thus, the Sun only appears to ‘wobble’ around this point.
Radial-velocity measurements work by applying this principle to distant stars. Observations are made of the spectrum of light emitted from a star, and the intensity of the light will vary over time depending on the ‘wobble’ of that celestial body. By measuring changes in radial velocity, not only the presence of an exoplanet can be determined but also its mass.
The problem with radial-velocity measurements is that they rely on the orbit of a potential exoplanet being directly in our line of sight. If the orbit is slightly tilted from the perceived horizontal plane, the star’s ‘wobble’ will be exaggerated, and so the planet’s mass will be overestimated. To counteract this, radial velocity is usually combined with astrometric observations, which track the motion of the star across the sky. Another flaw is that this method is only really useful for finding large planets orbiting close to a star, known as ‘hot Jupiters’. However, by partnering this with the transit method, it is possible to detect smaller Earth-like exoplanets.
This method of finding planets is based on the Gravitational lens effect. If a massive object, like a star, is directly in our line of sight of another massive body, then the star closest to us will bend and magnify the light of the background object. The two stars must be exactly aligned for this phenomenon to be observable, with the lensing event often lasting for no more than a few weeks. However, more than a thousand of these events have been observed in the past decade.
If there is an exoplanet In orbit around the star in the foreground, it will noticeably alter the lensing effect. This method is especially useful when peering towards the Centre of the Milky Way, as there are many background stars which produce this effect. Its main advantage is that it is able to observe exoplanets at a habitable distance from their host stars, but unfortunately other planet-hunting methods-will have difficulty confirming such planets St this distance.
Of all the methods listed here, gravitational microlensing is probably the least useful. This is because the lensing of two particular stars can never be observed twice, as they will never align in the same way again, thus making confirmation an incredibly difficult process.
In the future…Direct imaging
In the future, our best method of finding planets will be by directly imaging them. However, this is very difficult mainly as the planets themselves are so huge. Some modern telescopes, such as the ESO’s Very Large Telescope (VLT), are able to produce images of distant planets, although only large ‘hot Jupiters’. Indeed, In 2004 a team at the VLT was able to produce an image of 2M1207b, a planet several times larger than Jupiter in orbit around a brown dwarf. Future giant telescopes, such as the European Extremely Large Telescope (ELT), which has a mirror diameter of 39.3 metres (129 feet), might be able to observe distant exoplanets in greater detail, and even find some that are more akin to Earth.
Planet hunting telescope
SuperWASP is the UK’s main planet-hunting programme and is run by eight universities and institutions including Cambridge University. The programme consists of two near-identical telescopes. SuperWASP-North is located on La Palma, Canary Islands, and SuperWASP-South in Sutherland, South Africa. The telescopes found their first exoplanet on 26 September 2006, and since have found a further 66 exoplanets. The strength and size of the telescopes is such that they are unable to observe Earth-sized planets, but rather the gas giant types.
WASP-17b – One of the most interesting planets discovered by SuperWASP has been WASP-17b. This super-giant planet found in 2009 may be the largest in the universe (up to twice the size of Jupiter). It was also the first exoplanet discovered to have retrograde motion, which means that it orbits backwards relative to the spin of its host star.
The COROT (COnvection, Rotation and planetary Transits) space mission is run by French space agency CNES, the European Space Agency (ESA) and other global institutions. It is used to search for exoplanets and measure asteroselsmology In stars and is in orbit around Earth. Unlike NASA’s Kepler mission, which releases constant lists of candidate planets, the COROT team only announces the discovery of a planet once it has confirmed it Is an exoplanet. It found Its first two In 2007 – both hot Jupiter-like planets and has also located some super-Earths.
COROT-7b – This planet, located 489 light years from Earth in the Monoceros constellation, was first reported by COROT in February 2009. At the time it was the smallest-known exoplanet, just 1.58 times the size of Earth. It was the first potential rocky terrestrial planet to be found, unlike the gas giants that had previously been discovered.
Kepler is without a doubt the most successful planet-hunting telescope to date, and has been responsible for finding thousands of potential exoplanets. It was launched on 6 March 2009, and since then its planet candidate count stands at over 2,000; however, as of March 2012, it had only confirmed just over 60 planets. To find exoplanets the Kepler telescope employs the transit method, as discussed previously. The ultimate goal of the Kepler mission is to determine an approximate ratio of planets to stars, specifically those within habitable zones. Its observational mission is due to end later in 2012, although the multitude of data it has returned is nowhere near being fully analysed.
Kepler-20e – This was the first exoplanet smaller than Earth to be found, with a radius approximately 0.87 times that of our own world. Is has a mass almost equivalent to that of Earth, but it orbits its host star at a distance just 0.05 that compared to the Earth-Sun system, completing its orbit, and thus a year, in just six days. For this reason it is scorchingly hot – about 1.040K (767”C/1,410”F), and not capable of sustaining water or life as we know it.