Hiking, Skydiving and Booze: The Future of Exoplanet Tourism

 

Today, NASA released another poster in its wonderful ‘Exoplanet Travel Bureau‘ series. I’ve been a big fan of these prints since their inception; a fun and colorful outreach project that captures the diversity and exotic nature of these newly discovered worlds. However, they could be interpreted as more than just a NASA public relations project. If we consider art as a particular subjective interpretation of reality, subject to the fads, fashions and cultural context of the time, then these posters actually say quite a bit more!

They not only portray our early depictions of these enigmatic worlds, but our interpretations of what these planets may be like also sheds some light on our view of ourselves and the future. All the posters have a clearly evident retro-futuristic theme. A view of the future, but from the past. From the font choices, fashions and colour schemes, these posters are depicting the future of humanity, but through optimistic eyes of 1960s design, when flight and space exploration were developing at extraordinary rates, and holidaying on the Moon or Mars seemed only years away. By drawing on retrofuturism and space-age pop-art consumerism, the artists are trying to capture the anticipation and optimism of mid-20th century space science, and focus that enthusiasm on the hunt for Earth-like worlds, perhaps drawing parallels between the rate of planet discovery and that of aeronautics and space exploration during the 60s.

PSO J318.5-22 – Where the Nightlife Never Ends

The newest poster in the series depicts exoplanet PSO J318.5-22, a ‘rogue’ planet that has been ejected from its star system and is now sailing through interstellar space, but has since been turned into a party location by our intrepid descendants. To be honest, I really like this print. The throw-back to 1960s retro-futurism is undeniable, and this poster oozes cool and class – an elegant couple in minimalist spacesuits step out into the perpetual nightlife of a rogue exoplanet, a world so bizarre and strange that a few years ago the very idea of would have sounded preposterous. But here we are, turning an astronomical absurdity into a planetary-scale nightclub, all the while looking suave and beautiful as only humans can do. The simple duality of colors really capture the cold, cool and classy vibe, using blues, purples and silvery hues that gleam like auras off our descendants’ ‘nightclub-in-space’ regalia. In the background, figures stand suspended what appears to be a walkway along one of the planet’s icy rings, as more spaceships replete with party-goers arrives in sequence.

The three other posters in the series also adhere to a similar visual style. The print for Kepler-186 f depicts a couple exploring the striking red vegetation of this planet, the first Earth-size planet discovered in the potentially ‘habitable zone’ around another star. Its host star, Kepler 186, is a red dwarf, a much smaller star that emits more light in the red and infra-red portion of the electromagnetic spectrum than our Sun, and it is thought that if photosynthetic organisms were to exist on the surface of Kepler 186 f, they would be red in colour to exploit this fact. An interesting aspect of this poster is the white picket fence, a staple of American suburban utopia, that separates the green and red vegetation thereby accentuating the ‘grass is greener’ trope. Here, Earth-bound familiarity is juxtaposed against the alien backdrop of red foliage, and is proof that no matter how weird or exotic these worlds may be, the power of human culture will eventually make even the strangest of environments familiar to us.

When considering the artists’ interpretation of Kepler-16b, it is clear that the binary star system is the draw here. The unusual stellar architecture of this world would indeed make for a stunning double-sunset. Often considered a romantic activity best shared as a couple on Earth, here a single human figure enthralled by the graceful choreography of the binary is accompanied by two shadows instead.  Although likely much too cold for life, Kepler 16b is portrayed in familiar desert tones of ochra and red, perhaps reminiscent of planets in binary systems in popular culture.

Experience the Gravity of a Super Earth

So what if HD 40307 g is eight times as massive as the Earth? We can turn this to our advantage, and thrill-seeking human skydivers are invited to plunge at incredible speeds through its (probably) thick gaseous atmosphere. Why? Because we can. Because we’re humans and evidently the whole galaxy is our playground.

The fact we know relatively little about these planets actually helps in this case, as imagination and artist license can run rampant. Significantly, these prints frame these worlds as utilitarian or ‘useful’, but not in the traditional economic or scientific sense. They’re useful and subservient to a human species so technologically advanced that even our recreation and tourism is now planetary in scale. There’s not much science being depicted, and that’s fine. These are travel posters for the future middle-class, an invitation to come for the exotic sights and stay for the booze, hiking or skydiving. The limited depiction of ‘life’ in these posters is somewhat unimaginative – a few red trees and the tinge of green on a continent, but I think that’s for the best at this stage. Overly stylized images of alien life would detract from the fact that humans are as much the focus in these posters as the alien planets.

The fact that we’re portrayed as out there using entire worlds for our recreation and fun speaks to the optimism of the search for extrasolar planets, and is well mirrored by the retro style employed by the artists. Unfortunately, being able to travel to these worlds is, in reality, completely unfeasible with contemporary technology, which makes the promises of lavish parties in deep space somewhat bittersweet. Nevertheless, I hope that NASA continues to publish posters in this series, and I look forward to seeing their next effort!

 

 

 

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The hunt for an Exo-Earth: How close are we?

This is a guest post by Hugh Osborn, a PhD student in the Astronomy and Astrophysics group at the University of Warwick. Hugh’s research involves using transit surveys to discover exoplanets. Visit his excellent blog, Lost in Transitsfor more on exoplanets, their detection and his research.

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In the 1890s Percival Lovell pointed the huge, 24-inch Alvan Clark telescope in Flagstaff, Arizona towards the planet Mars. Ever the romantic, he longed to find some sign of life on the Red Planet: to hold a mirror up to the empty sky above and find a planet that looked a little bit like home. Of course, in Lovell’s case, it was the telescope itself that gave the impression of life, imposing faint lines onto the image that he mistook for canals. But, with Mars long since relegated to the status of a dusty, hostile world, that ideal of finding such a planet still lingers. In the great loneliness of space, our species yearns to find a world like our own, maybe even a world that some other lineage of life might call home.

51 Pegasi: Home to the first exoplanet discovered by humans (Copyright: Royal Observatory Edinburgh, Anglo-Australian Observatory, and AURA)

A hundred years after Lovell’s wayward romanticism, the real search for Earth-like planets began. A team of astronomers at the University of Geneva used precise spectroscopy to discover a Jupiter-sized world around the star 55-Peg. This was followed by a series of similar worlds; all distinctly alien with huge gas giants orbiting perishingly close to their stars. However, as techniques improved and more time & money was invested on exoplanet astronomy, that initial trickle of new worlds soon turned into a flood. By 2008 more than 300 planets had been discovered including many multi-planet systems and a handful of potentially rocky planets around low-mass stars. However, the ultimate goal of finding Earth-like planets still seemed an impossible dream.

In 2009 the phenomenally sensitive Kepler mission launched. Here was a mission that might finally discover Earth-sized planets around Sun-like stars, detecting the faint dip in light as they passed between their star and us. Four years, 3500 planetary candidates and 200 confirmed planets later, the mission was universally declared a success. Its remarkable achievements include a handful of new terrestrial worlds, such as Kepler-61b and 62e, orbiting safely within their star’s habitable zones. However, despite lots of column inches and speculation, are these planets really the Earth 2.0s we were sold?

While such worlds may well have surfaces with beautifully Earth-like temperatures, there are a number of problems with calling such worlds definitive Earth twins. For a start the majority of these potentially habitable planets (such as Kepler-62e) orbit low-mass M-type stars. These are dimmer and redder than our Sun and, due to the relative distance of the habitable zone, such planets are likely to be tidally locked. The nature of such stars also makes them significantly more active, producing more atmosphere-stripping UV radiation. This means, despite appearances, ‘habitable’ planets around M-dwarfs are almost certainly less conducive to life than more sun-like stars.

Even more damning is the size of these planets. Rather than being truly Earth-like, the crop of currently known ‘Habitable planets’ are all super-Earths. In the case of Kepler’s goldilocks worlds, this means they have radii between 1.6 and 2.3 times that of Earth. That may not sound too bad, but the mass of each planet scales with the volume. That means, when compression due to gravity is taken into account, for such planets to be rocky they would need masses between 8 and 30 times that of Earth. With 10ME often used as the likely limit of terrestrial planets, can we really call such planets Earth-like. In fact, a recent study of super-Earths put the maximum theoretical radius for a rocky planet as between 1.5 and 1.8RE, with most worlds above this size likely being more like Mini-Neptunes.

So it appears our crop of habitable super-Earths may not be as life-friendly as previously thought. But it is true that deep in Kepler’s 3500 candidates a true Earth-like planet may lurk. However the majority of Kepler’s candidates orbit distant, dim stars. This means the hope of confirming these worlds by other techniques, especially tiny exo-Earths, is increasingly unlikely. And with Kepler’s primary mission now ended by a technical fault, an obvious question arises: just when and how will we find a true Earth analogue?

Future exoplanet missions may well be numerous, but are they cut out to discover a true Earth-like planet? The recently launched Gaia spacecraft, for example, will discover hundreds of Gas Giants orbiting Sun-like stars using the astrometry technique, but it would need to be around a hundred times more sensitive to discover Earths. New ground-based transit surveys such as NGTS are set to be an order of magnitude better than previous such surveys, but still these will only be able to find super-Earth or Neptune-sized worlds.

The Transiting Exoplanet Survey Satellite (TESS) (space.mit.edu)

Similarly, Kepler’s successor, the Transiting Exoplanet Survey Satellite which is due to be launched in 2017, will only be able to find short-period planets with radii more than 50% larger than Earth. HARPS, the most prolific exoplanet-hunting instrument to date, is also due for an upgrade by 2017. Its protégée is a spectrometer named ESPRESSO that will be able to measure the change in velocity of a star down to a mere 10cms-1. Even this ridiculous level of accuracy is still not sufficient to detect the 8cms-1 effect Earth’s mass has on the Sun.

So despite billions spent on the next generation of planet-finders, they all fall short of finding that elusive second Earth. What, precisely, will it take to find this particular Holy Grail? There is some hope that the E-ELT (European-Extremely Large Telescope), with its 35m of collecting area and world-beating instruments will be able to detect exo-earths. Not only will its radial velocity measurements likely be sensitive enough to find such planets, it may also be able to directly image earth-analogues around the nearest stars. However, with observing time likely to be at a premium, the long-duration observations required to find and study exo-earths could prove difficult.

Alternatively, large space telescopes could be the answer. JWST will be able to do innovative exoplanet research including taking direct images of long-period planets and accurate atmospheric spectra of transiting super-Earths and giants. Even more remarkably, it may manage to take spectra of habitable zone super-Earths such as GJ 581d. But direct detection of true Earth-analogues remains out of reach. An even more ambitious project may be required, such as TPF or Darwin. These were a pair of proposals that could have directly imaged nearby stars to discover Earth-like planets. However, with both projects long since shelved by their respective space agencies, the future doesn’t look so bright for Earth-hunting telescopes.

After the unabashed confidence of the Kepler era, the idea that no Earth-like planet discovery is on the horizon may come as a surprisingly pessimistic conclusion. However not all hope is lost. The pace of technological advancement is quickening. Instruments such as TESS, Espresso, E-ELT and JWST are already being built. These missions may not be perfectly designed to the technical challenge of discovering truly Earth-like planets, but they will get us closer than ever before. As a civilisation we have waited hundreds of years for such a discovery; I’m sure we can hold out for a few more.

Detection and Discovery of Exoplanets

thingswedontknowThis the second in a series of posts by me at Things We Don’t Know about the many unknowns involved in the study of planets in the orbit of other stars across the galaxy.

The first planet discovered orbiting another star was detected by astronomers at an observatory in France in 1995. The planet is an enormous gas giant, half the mass of Jupiter, orbiting very close to the Sun-like star 51 Pegasi in the constellation Pegasus, 50 light-years from Earth. The existence of other planetary systems had been predicted by astronomers for centuries and the discovery marked a monumental breakthrough in astronomical research. Since then, rapid improvements in technology and observational techniques have resulted in the discovery of 863 confirmed ‘exoplanets’ to date.

How many planets are there? As astronomers hunt for planets orbiting other stars, we are starting to form a picture of  how many planets there are in the galaxy. Image credit: Luke Surl, for TWDK

How many planets are there? As astronomers hunt for planets orbiting other stars, we are starting to form a picture of how many planets there are in the galaxy. Image credit: Luke Surl

Unlike the direct observation of stars, the detection of planetary bodies requires astronomers to use a number of indirect methods to infer their existence. Due to the immense distances involved, the distance between any planet and their host star when viewed from Earth is tiny, and the brightness of the star itself effectively blinds instruments and obscures any planets in their orbit, which are much less bright by comparison. Therefore, astronomers have devised a number of ingenious methods to tease out planet data from their observations, but they require a great deal of skill, a generous helping of statistical analysis and a pinch of luck.

The most successful means of planet detection to date, yielding roughly 58% of all discoveries, is called the radial velocity method. This technique exploits the fact that the host star and its planets orbit a common centre of mass, and the planets exert a tiny ‘tug’ on the star that results in a very slight wobble – a signature that can be detected and used to infer the existence of one or more planets. Another successful indirect method of detection, responsible for a third of exoplanet discoveries, is called the transit method. When viewed from the Earth, a planet orbiting a star periodically passes in front of the star (‘transits’) and obscures a very small amount of its light, resulting in a tiny but consistent reduction in the amount of light received by Earth-based instruments. The amount of light that is blocked out provides some information about the size of planet, as larger planets will obscure relatively more light, and the frequency and duration of the transit can be used to infer the distance from the star that the planet orbits. NASA’s Kepler space telescope, launched in 2009, uses this method and it has proved extremely fruitful, resulting in the discovery of 105 confirmed exoplanets to date. Additionally, there are a further 2,740 potential planets (called ‘planet candidates’) detected by Kepler awaiting confirmation.

Kepler’s search area extends 3000 light years from Earth along the Orion Spur of the Milky Way. Image copyright © Jon Lomberg, used with permission.

However, the science of exoplanet detection is by no means certain; many teams use different statistical methods to isolate exoplanet signals, and the lack of consistency means that many discoveries are initially met with scepticism. With little means of directly imaging these planets, debate continues about the existence of a number of exoplanet candidates, and the finer details of many confirmed planetary systems. Also, the methods mentioned above tend to favour large planets as their effect on their star (either by increased ‘wobble’ or by concealing more light during transit) is proportionally greater.

We find ourselves at an exciting, but also frustrating, juncture at the birth of exoplanet detection. Our 862 planet sample is impressive and the effort and skill of the astronomers responsible for their detection should be applauded. However, we have only begun to scratch the surface of planet discovery. Kepler can survey an impressive 100,000 stars, but that is only one millionth of the total stars in the Milky Way Galaxy. Many, many more stars and planets remain out of reach of our telescopes, at least for the foreseeable future.

First direct picture of an alien planet orbiting a Sun-like star. Image credit: Gemini Observatory

Admittedly, to say that no planet has been directly imaged would not be quite accurate. Some extremely large planets, in most cases 5 or 10 times the mass of Jupiter, orbiting at great distances from their stars have been directly imaged. These first pictures represent great steps forward for exoplanet research, but technological constraints impose limits on the size and orbital distance of planets able to be imaged in this way, and the direct imaging of small, Earth-like planets orbiting relatively near to their host stars is not yet possible.

In my next post, I hope to take a more detailed tour through the current exoplanet catalogue to highlight some of the interesting and exotic planets that inhabit our galactic neighbourhood, and illustrate what the diversity of these planets can tell us about the Earth and our Solar System.