Tag Archives: Habitable Zone

Looking for Life – Part I

Astrobiology, , , , , , , ,

“Why do astronomers look for Earth-like planets in their hunt for life in the universe. Why couldn’t life exist in all manner of ways that we are incapable of contemplating?”

I see this question, put in a variety of different ways, all the time. And to first glance, it is a valid question. Why should we focus on finding Earths? What if, by searching for only the ‘twin’ of Earthlife, we’re missing a plethora of friends and family? I hold the opinion that we’re not. That life formed of carbon, oxygen & hydrogen atoms suspended in water is in all likelihood the most common, and maybe the only, way that the universe is able to ‘do’ life. And that, while the habitable zone (the zone where an Earthlike planet would have liquid water on it’s surface) is not the only place in the universe that these ingredients can get together, it is both most likely and most likely to be detectable there.

So I will lay out my argument here in a series of posts that are almost certainly better explained elsewhere.

What is life?

A question famously posed by Schrödinger in the 1940s, it is also the first key question on our path: To find life, we need to know what we’re looking for…

Defining life may, to first glance, seem obvious. But the boundary between the living and the lifeless is hard to define – just ask any virologist. Definitions of life tend to focus on the fact that all lifeforms are structures capable of self-replication (ie reproduction), movement, growth, and the ability to evolve. But these properties, themselves, are not unique among the natural world. Crystals can replicate themselves and even respond to the environment. Bubbles of fat can inorganically grow and replicate. And anything capable of near-perfect self-replication is likely to feel the process of natural selection.

One of Schrödinger’s great insights about Life is that it does not die. It “evades the decay to equilibrium”, instead exchanging material with the environment to keep alive for much longer than an ordinary lump of matter might. In Schrödinger’s words, life feeds on ‘negative entropy’ (order in the environment) and produces positive entropy (in waste and heat).

But just how valid is this definition? Are we focussing too much on earth-life and leaving out a whole universe of possibilities. Could life be simple, rather than complex; electrical rather than chemical; or even gaseous rather than semi-solid? Give each point a bit of thought and the alternative, to my mind, sound implausible:

Complex – Picture an airless world covered in fine, unstable sand. One day, maybe thanks to a meteorite hit, a cavity is created and a small cascade begins, slowly eating its way into the surface like a wave. I can see this cascade steadily feeding off the gravitational energy of the raised sand, increasing entropy in the low sand left behind. It might even grow over time and then split into two or more such cascades. But is this system alive? My thought would be ‘no’, because of the final argument for life – that it must undergo evolution by natural selection. A single simple process such as this, while ticking all the boxes for life, cannot adapt. The cascade of sand is not determined by any in-built mechanics, but simply the result of disequilibrium.

There is also no easy way to turn one molecule into another and energy. Even the simplest chemical pathways have an energy barrier to get over, such that stored energy and catalysts are needed just to make more energy and avoid death. Then there is the apparatus needed to reproduce not just all that energy-making machinery, but also the structures that hold this all in one place. Life, by its very definition, seems to need to be more complex that the environment it feeds on to evolve.

Chemical – The universe has plenty of other ways of making and transferring energy, as the example above shows. But life requires not only creation of energy, but also to store and use that energy too. While electrical, thermal or even light energy could likely be used (indeed life on earth does), this energy cannot itself form its own storage facility; Light cannot conjure its own mirror. For that, it needs to interact with chemistry. Only the chemistry of complex molecules has the dexterity to perform these storage and reproduction tasks.

Semi-solid – Specifically, life must have a barrier to it’s environment. Just to avoid death and decay, all life on earth continually cycles through its own body weight in energy and resources once every day or so. This is only possible thanks to the fact that our bodies have a semi-solid barrier to the surroundings, with the molecules of metabolism able to leave by diffusion. If metabolism excreted solids (like FeS or SiO), these products would build up and suffocate the organism. If an organism were fully liquid or gaseous, all the important self-replicating machinery would drift away due to diffusion. Life needs to be self-contained, and it likely needs a liquid solvent in which to live…

I hope I’ve shown you that, by simply starting at the definition of life and applying physics, we can get a decent handle on what life in the universe must be like. It seems like that life must be more complex that it’s surrounding; it must be a chiefly chemical system and it must be self-contained. That is a surprisingly narrow conclusion, but surely there could be a myriad of ways to assemble a chemical system such that it fits the criteria for life? We shall explore that next in “The Ingredients of Life”.

 

Further Reading:

Erwin Schrodinger – What is Life (pdf version here)

Nick Lane – The Vital Question (review)

Gliese 581d is an ex-planet

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Gliese581TopTrump
Exoplanet poster child

If, in 2009, you asked 18-year-old me to name an exoplanet, then Gliese 581d would have been it. Discovered by an American team of astronomers in 2007, it was, for a long time, the poster child for exoplanetary science. Not only was the first rocky world ever found in the habitable zone of its star where life-friendly temperatures are found, it was also relatively nearby (for astronomy standards) at only 20 light years.

Astronomers used the radial velocity technique to find the first planet around Gliese 581 as far back as 2005. This method relies on the gravitational pull that a planet has on a star as it orbits. This wobble is detectable in the spectra of the starlight, which gets doppler shifted as the star moves back-and-forth, allowing the period and mass of an orbiting planet to be determined. While the first planet, ‘b’, orbited close to the star with a period of only 5.4 days, it was joined by two cooler (and more habitable) planets, ‘c’ and ‘d’ in 2007. This was soon followed in 2009 by Gliese 581e, the smallest planet in the system on an even shorter (3.1d) orbit.

RVgif
Movie credit: ESO

Things started to get even more confusing in 2010 when observers at the Keck observatory announced two more planets (‘f’ and ‘g’) orbiting at 433 and 37 days respectively. This would put ‘g’ between ‘c’ and ‘d’ and right in the middle of the star’s habitable zone. However, new observations of the star with a Swiss telescope showed no such signal. Was there a problem with the data, or could something else be mimicking these planets?

article-2003824-006ED8C000000258-933_634x565[1]
Other stars, just like our sun, have extremely active surfaces
One problem comes when we consider the star itself. Just like our own sun, most stars are active, with starspots skimming across the surface and convection currents in the photosphere causing noise in our measurements. These active regions can often mimic a planet, suppressing the light from one side of the rotating star and shifting the spectra as if the star itself were moving back-and-forth. Add to that the fact that, like planets, activity comes and goes on regular timescales and that cool stars such as Gliese 581 are even more dynamic than our pot-marked sun, and the problem becomes apparent.

The first planet to bite the interstellar dust was ‘f’. At 433 days, its orbit closely matches an alias of the star’s 4.5-year activity cycle, and it was quickly retracted in 2010. Similar analyses with more data also suggested Gliese 581g was also likely to be an imposter, but the original team stuck by this discovery. For the last 3 years, this controversy has simmered, until last month all the data available for Glises-581 was re-analysed by Paul Robertson at Penn State. This showed that not only is Gliese 581g not a planet, but that the poster child itself, Gliese 581d, was also an imposter.

CorrectedPeriodogram
The signal strength of any potential planets with (red) and without (blue) activity correction.

To do this, the team took all 239 spectra of GJ581 and analysed not just the apparent shift in velocity, but the atomic absorption lines themselves. Using the strength of the Hα absorption line as an indicator for the star’s activity, they compared this to the residual radial velocity (after removing the signal from planet b). This showed that there was a relatively strong correlation between activity and RV, especially over three observing seasons when the star was in a more active phase. They also found that this activity indicator varied on a 130 day timescale.

581_orbits[1]
The new system with only 3 planets
When the team removed the signal from stellar activity, they found that planets ‘c’ and ‘e’ were even more obvious than in previous searches. However the signal for planet ‘d’ dropped by more than 60%, way below the threshold needed to confirm a planet. Even more remarkably, ‘g’ does not appear at all. So what exactly caused this ghostly signal. The planet’s orbital period of 66 days gives us a clue -it is almost exactly half that of the star’s 130 day rotation cycle, so with a few fleeting starspots and the right orientation, a strong planet-like signal at 66 days results.

This case of mistaken identity is a sad one, but thanks to the incredible progress of our field in the last 5 years, their loss barely makes a dent in the number of potentially habitable exoplanets known. Instead, it acts as a warning for planet-hunters: sometimes not all that glitters is gold.

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The results are also explained in exquisite detail at Penn State University’s own blog, including an excellent timelapse showing how our understanding of the Gl 581 system has changed over time

Goldilocks Worlds: An Infographic

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NatGeoInfographic
Full, clickable “Goldilocks Worlds” infographic available from National Geographic here

Today the National Geographic released an extremely interesting infographic on exoplanets. It shows all 1000 confirmed planets and 700 validated Kepler Candidates and their vital statitistics. The x-axis gives the amount of light from it’s star, the y-axis has mass, each planet’s radius can be seen from the size of each point and the clickable version even displays each planet’s name. Most prominent on the diagram is the Goldilocks square containing a handful of  exoplanets “just right” for life along with Earth and Mars.

Credit: Planetary Habitability Lab
Credit: Planetary Habitability Lab

As an infographic it is a beautiful and succinct way of showing what we know about planets around other stars. It makes two key facts about exoplanet detection plainly obvious: that most of the planets we currently know are big and hot; and that despite these limitations we are gradually pushing towards the detection of habitable, Earth-like planets. The position of newly-discovered Kepler 186f, a centimetre or so to the right of Earth, is testament to that.

But how does the science itself hold up? Well, as any good science teacher will say, always label your axes and use error bars. But we can let that slide as it is an infographic and not an undergraduate project.

MassvsRadiusRelation
Mass and Radius just don’t get along (from Butler & Marcy, 2014)

How about the position of each point though? Well, the graph uses planetary mass as y-axis parameter. However for almost all of the low-mass planets displayed (ie. the Kepler candidates) the mass is almost completely unknown. All is known is the radius, and this can be used to give a rough estimate of the mass. And when I say rough, I mean extremely rough. For each radius value selected, the range of potential masses varies by more than 3 Earth masses even for Earth-sized planets! That could push a planet currently within the “Just right” square such as Kepler 283c into the ‘too large’ area and vice-versa.

Even that box should not be taken as given. The idea of a habitable zone varying with its distance from a star makes sense: too hot and water begins to boil away. Too cold and it freezes. But there are a huge number of things that could change those limits including tidal locking, atmospheric composition, surface reflectivity, atmospheric density, etc. To account for all of these is almost impossible, and to plot them all on a 2D plot certainly is. Current models (and the vertical lines you see here) get around this by assuming almost every parameter is Earth-like. For different sized planets, or those with unusual atmospheres, that assumption could break down (although work is certainly being done).

KopparapuHZmasses2014
The variation of habitable zone with planet mass (Kopparapu et al, http://arxiv.org/pdf/1404.5292 )

The habitable limits of planetary mass are even more like guesswork. Certainly, gas giants and tiny asteroids would appear less habitable than Earth mass planets, but the position of the limits at 0.1 and 10Me are arbitrary. There is no real reason why a large super-Earth or small sub-Mars could not support life, and certainly very little science has so far been done on this area so far.

So, despite displaying the main information well, this infographic gives the impression that we know a lot more than we actually do about both the limits of life and the characteristics of the planets that could hold it.

But, as was pointed out on twitter, this is not a scientifically published figure, but an infographic. It is something designed to spread knowledge in its simplest, uncomplicated form. And for that, it is fantastic.

Kepler’s Last Stand: Verification by Multiplicity

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TNG_LaPalmaFor 3 months a year, the TNG telescope on the island of La Palma turns its high-precision spectrometer (HARPS-N) towards the constellations of Cygnus and Lyra. This is the field of view that NASA’s Kepler space telescope stared at for more than 3 years, detecting thousands of potential new exoplanets using the transit method. There the TNG scans hundreds of Kepler’s potentially planet-holding stars looking for tiny changes in their radial velocity. If detected, this signal will indicate the presence of a real planet, confirming once and for all what Kepler first hinted at many months before. This is the process that, up until now, has been used to definitively find the majority of Kepler’s 211 planets.

exoplanetdiscoverieshistogram
New ‘discoveries’ in context

That appeared to change in the blink of an eye this week with the confirmation of 715 new planets using a new catch-all statistical technique. But how did the Kepler team confirm all these new worlds, and can they really be considered real planets?

Without further observations with instruments such as HARPS, Kepler’s 3000 planetary candidates cannot usually be called definite planets. This is because a number of other signals could mimic the transit signal of a star, including tightly bound double-stars that graze one other as they orbit, or unseen dim stars that have binary companions. Alternatively the cameras themselves could be acting up, producing periodic, transit-like signals in the data. Last year a team used simulations of the Kepler data to estimate that around 10% of the candidates were likely to be such false positives.

KepCands
Kepler Candidates by size

So how can more than 700 worlds be confirmed at once, without any manual work from telescopes on the ground? The answer is through performing statistics on Kepler’s planets. Of a zoo of 190,000 stars observed, Kepler discovered 3000 potential planets, of which 10% are likely to be spurious signals. As a rough estimate then (and the Kepler team go into much more effort than this), the random probability of finding a false positive is 300/190,000, or a rate of only 0.16%.

That number on its own cannot help confirm planets. The trick comes when thinking about Kepler’s hundreds of multiple planet systems. The likelihood of a single-planet system randomly having another false positive also in the data is extremely low. In fact, applying that rough number to the 1000 best single-planet candidates tells us only around 2 of those multiplanet systems should have a spurious planet. Similar calculations can be done for even rarer systems with two false positives, two planets and a false positive, etc.

KeplerFPs
Possible False Positve Signals

This rate can also be significantly improved by excluding any targets more likely to give these spurious signals. For example, the authors removed more than 350 potential planets from the initial sample for many reasons. Some had instrumental artefacts seen in other stars or had transits close to the limit of detection. Others with V-shaped transits were eliminated as these are more likely to be grazing binary stars. The team also studied the images Kepler took to check for possible transits on a secondary star, eliminating anything where the transit did not in the star’s central position.

Using these cuts, the study narrowed down the search to 851 planets around 340 stars. Applying statistics and using the estimate that 10% of currently detected planets might be false positives, the team found that 849 of the 851 planets were likely to be planets. This corresponds to a certainty of 99.8%,  just greater than 3σ, which in astronomy is usually enough to constitute a detection. This is how “verification by multiplicity” works.

sizes
Confirmed Kepler Planets by Size

Of these, 715 are previously un-confirmed worlds. Nearly all are relatively small planets, with radii going from the same as Earth up to that of Neptune. Four of these new planets may also reside in their star’s habitable zone, the region where liquid water could exist on the surface.

As amazing as it would be to nearly double the number of exoplanets overnight, some doubts remain about this method. By eliminating astronomical follow-ups, no extra information can be gleaned. For example, without performing radial velocity measurements, the mass of these planets will never be known. And without other accurate astronomical studies, we cannot accurately determine the nature of the star, and therefore the radius of the planet.

The main difference, though, comes from the impersonal nature of verification by multiplicity. Previous confirmation methods assessed the probability of each candidate being a planet individually. By performing the confirmation in bulk we will know, thanks to the statistics, that at least 2 planets are imposters*. But if exoplanet astronomers can learn to live with that doubt, such planets may well be accepted as confirmed worlds and this simple idea will see the single biggest influx of validated exoplanets in history.

* Here’s another way to compare those statements. Imagine you have two pills. One produces a 0.2% chance of death. The other causes the loss of two fingers (0.2% body mass). By adding these planets to the list of exoplanets, we may well gain a whole new body of worlds, but there will be painful amputations to come in the future.

The two papers, which will be released on March 10th in ApJ, can be found here (Lissauer, 2014) and here (Rowe, 2014).

UPDATE: The new planets are proving reasonably contentious. The exoplanet counter on NASA’s planetquest sits at 1690 , wheras the Paris-based exoplanet.eu remains on 1078. Time will tell whether astronomers accept these as true planets or simply string candidates.

What can PLATO do for exoplanet astronomy?

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As readers of my previous post will no doubt know; the future looks grim for exoplanetary science. Kepler is dead, Hubble will soon follow and we face a long wait before the next generation of planet-hunting instruments. But this week, exoplanet astronomers glimpsed another ray of hope. The next £500million of European Space Agency money looks likely to go to PLATO; an incredible exoplanet-hunting mission set to be even better than Kepler.

PlatoConcept

With an array of 34 telescopes mounted on a sun-shield, PLATO hopes to do things a little differently from both Kepler and TESS. Like those missions, it too will monitor thousands of stars looking for the minute dip in light caused by the passage of a planet in front of its parent star. However, it is in both breadth and depth that PLATO excels; with the combined light of dozens of cameras allowing 5% of the sky to be monitored to incredible accuracy at any one time.

Platofield
Plato’s likely field of view, with 2-3yr stops in red

More than a million stars could be scrutinised for Earth-sized planets by Plato, giving an expected planet haul an order of magnitude higher than Kepler. Plato will also not be tied down into staring at the same stars, instead monitoring 50% of the sky on eight 30-day positions and two longer 3-year fields. This will allow dozens of Earth-like planets with potentially habitable temperatures to be discovered.

The main criticism of the now-defunct Kepler mission was the faintness of these stars (between magnitude 7 and 17). This meant the vast majority of its planetary candidates were impossible to follow up and confirm. The wide field and large array of cameras on Plato allow the brightest stars to be monitored (mag 4-16). That will mean even tiny Earth-sized worlds found by Plato can be followed up and confirmed by ground-based telescopes.

Astroseismology
The concept of Astroseismology

This ability to survey bright stars also allows astronomers to perform extremely sensitive measurements of the stars themselves. By using variations in starlight caused by ripples on the star’s surface, astronomers can accurately pin down not only the size of the star but also the age of the star system. This means, not only can Plato find exoplanets around bright stars, but it can also determine the size and age of many of these planets to a precision only previously dreamed of.

The Transiting Exoplanet Survey Satellite (TESS), to launch in 2017, seems superficially to be a similar mission to Plato. It will potentially discover hundreds of planets before Plato even gets off the ground in 2024. However, the limited sensitivity of its cameras mean it is completely blind to Earth-like worlds around sun-like stars. Astroseismology is also off-limits for TESS, meaning the size of any worlds it does discover will be highly uncertain. Unlike Plato, it will also move between patches of sky every 30 days, allowing only hot, short-period planets to be found.

The-Earth-Moon-System
The only truly habitable planet yet known

With all other new telescopes, both in space and on the ground, limited to finding super-Earths around small stars, Plato is the only mission on the table truly capable of discovering an Earth-like world around a star like our Sun. And by targeting bright stars that allow atmospheric follow-up, it is not impossible to think that, as well as the first truly habitable planet, Plato could find the first inhabited one too.

However, the decision process for ESA’s Cosmic vision (M3 class) is still ongoing. It would be highly unusual for ESA member states to overturn the mission recommended by the science committee, but in the political cauldron that is ESA anything is possible. If Plato does get through unscathed, it will bring riches not just to the universities, countries and industries involved, but more significantly to the world of science as a whole.

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The paper detailing mission design and expected science results can be found at: http://arxiv.org/abs/1310.0696 . The official ESA mission page has similar information at: http://sci.esa.int/plato/

PlatoCandidates
A comparison of Kepler Candidate planets (the majority of which are too faint for follow-up observations) against likely PLATO candidates (significantly brighter, eg with a lower magnitude)

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

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This blog was first published as a guest post on Andrew Rushby’s excellent II-I- blog.

Lowell_Mars_channels

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.

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.

Kepler-62f_with_62e_as_Morning_Star

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?

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.

TESS_satellite (1)

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 not sufficient to detect the 8cms-1 effect Earth’s mass has on the Sun.

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 and late K-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.

e-elt-1_2008

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.

2013 in Exoplanets

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The last year has been an extraordinary one for exoplanetary science. With nearly 200 new planets found, and thousands of scientific papers produced, it was tough work narrowing down such a year to a highlights reel. But, without further ado, here is my Top 10 discoveries from the past 12 months:

10. New Habitable Zone Definitions

HabitableZone2

In early 2013 a new team took a fresh look at the calculations defining the habitable zone, that hypothetical band around every star that an Earth-like planet would have liquid water on the surface. Using modern data, they redefined the distances and suggested that earth was closer to the dangerous inner edge than previously thought. This led to other interesting habitable-zone related news that I have left out to avoid accusation of vanity.

9. Three habitable worlds around GJ667C

800px-Gliese_667

Radial Velocity measurements of the star GJ667C found two more potentially habitable planets around the smallest member of this unusual triple star system. All three of these planets, c, e and f, made the Habitable Exoplanet Catalogue’s list of the most life-friendly worlds.

8. Seven-planet solar system found

KOI-351

Reanalysis of the Kepler data by a variety of teams (including the Planet Hunters citizen science project) added a seventh planet to the six worlds already known to circle Kepler-90. With all seven planets circling within the orbit of Earth this not only makes the most numerous planetary system, but also the most compact.

7. ‘Family portrait’ spectra of 3 hot young Jupiters

HR8799crop

Studying the atmospheres of exoplanets is extremely tricky business. But a single measurement with the Hale telescope in California was able to take a peek at the atmospheres of three young planets around the star HR 8799. The team found the distinctive signature of methane in the atmospheric spectra of all three worlds as well as a tentative detection of either ammonia (NH3) or acetylene (C2H2).

6. Kepler and CoRoT die

A review of the year would not be complete without taking a moment to consider those lost.

Kepler-telescope

In May a fault with Kepler’s third reaction wheel left it seriously wounded. This problem means, after 4 years of service and 3500 planetary candidates, its primary mission is over. Despite its injury, Kepler will still be able to contribute to space science and a secondary mission is due to be chosen in early 2014.

July saw ESA’s CoRoT mission pronounced officially dead. This transit-observing spacecraft suffered a major computer fault in November after 6 years of dedicated service and was unable to be resuscitated.

5. TESS selected

TESS_satellite

With death comes new life, and new missions launched and proposed in 2013 look certain to take up the mantle of Kepler and CoRoT.

In April, the Transiting Exoplanet Survey Satellite (TESS) was selected by NASA to launch in 2017. Unlike Kepler, it will scan the entire sky looking for exoplanetary transits, and potentially find hundreds of small rocky exoplanets around nearby stars.

4. Gaia Launched

gaia

Blasting off on board a Soyuz rocket from Guyana in December was Gaia. The most sensitive camera ever sent into space to do astronomy, the space telescope will look for the subtle motion of stars due to planetary companions. If all goes to plan by 2020 it will have found another thousand gas giants to add the current crop of exoplanets.

3. Three habitable worlds found by Kepler

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April saw NASA announce evidence for three new planets discovered by the Kepler mission. These were not the usual crop of Jupiter-like worlds, however. The planets were some of the most Earth-like yet found. Two of these were found in the Kepler -62, with both e and f orbiting within the star’s Habitable Zone and with radii only 40% and 60% larger than Earth’s respectively. While Kepler-62 is a dim dwarf star, Kepler-69c circles a G-type star similar to our own Sun and, once again, the 1.7RE planet was found within the habitable zone.

2. 1000 exoplanets and WASP’s 100th

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In October the number of exoplanets recorded by the exoplanet.eu database ticked over 1000 after the announcement of more than a dozen planets found by the WASP transit survey. These planets included WASP-100 and 101, giving WASP a similar milestone and making the planet-hunting telescopes by far the most successful ground-based transit survey.  By the end of the year, the counter stood at 1056, with a total of 188 new planets added in 2013 alone, and it is likely that 2014 will see even more new planets discovered.

1.  An Earth-like planet is only 13ly away

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2013 saw a fundamental question about the universe finally answered: How far do we have to go before we find a planet that looks like home? Thanks to the wealth of data from Kepler, astronomers were able to definitively say: 13 light years.

By looking at the number of Earth-like worlds in the habitable zone of M-Dwarfs, the most common stars in the galaxy, the Kepler team were able to estimate that 6% of such stars will have their own Earths orbiting them. And while 13 light years is probably far too far away for a visit, proposed telescopes will be able to take a closer look, potentially even hunting for signs of life.

Habitable Lifetimes: 50 Billion Years of Summer

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For 4 billion years our planet has been a willing host to life; nurturing it as it evolved from the first primitive single celled organisms through to large, intelligent life forms such as ourselves. Over time our sun, too, has evolved; growing in brightness by perhaps as much as 30%. And someday in the distant future Earth’s long glorious summer will end; our fuel-hungry sun glowing ever brighter until the planet we call home is scorched beyond recognition.

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Media favourite: a dead, uninhabited Earth (in 4bn years)

That is certainly a disappointing conclusion for us Earth-dwellers, but not exactly the one myself and colleagues at the University of East Anglia came up with in a paper published in Astrobiology this morning (despite the mainstream news outlets you might have read).

The slow expansion of our sun has long been predicted by astrophysicists, who revealed the clockwork of stellar evolution as far back as the 1970s. Other developments in the 1990s confirmed this by estimating the range of distances from the sun (and hence temperatures) over which an Earth-like planet would retain liquid water at the surface. The idea of this Habitable Zone has since been the go-to tool for assessing whether a planet could support life, and for as long as it has existed it has been known that the Earth is edging closer and closer to the too-hot-for-life ‘inner edge’.

By using recent models of how stars expand and brighten over time, we were able to put a new (if somewhat uncertain) estimate on when such a transition might happen: between 1.75bn to 3.25bn years from now. But while that might be as far as the papers read, the real science goes much deeper…

By the time Earth is toast, our blue planet will have dwelled for between 5 and 7 billion years in this glorious goldilocks zone. This is the Habitable Lifetime, and by anyone’s standards it is astoundingly long. Without it, life on Earth would have never had time to evolve from inorganic soup into the wonderful range of complex and intelligent creatures we see today.

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Numerous habitable zone planets have now been discovered

But Earth is not the only potentially life-supporting planet out there, and instead our research was focused on how long these other planets might remain habitable. Before the sun had brightened, Venus may have enjoyed 1.3bn years of balmy temperatures, while Mars may spend a few billion years bathing in similar sunshine near the end of the sun’s 10bn year lifetime. Almost 1000 alien planets have also now been found including a handful near their star’s habitable zone, not to mention a further 3000 Kepler candidates waiting in the wings.HabLifetimes

Computing the habitable lifetimes of these exoplanets is a more difficult task, however, as every star evolves at a different rate. Luckily stars only change brightness based on one thing: their size, and this can be found for the majority of stars. The 34 planets produce a large range of habitable lifetimes from 0.1 to 20bn years. One particular case is Kepler-22b which will remain in the habitable zone for 4.3bn and 6.1bn years; almost the same as Earth.

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All stars <35% the size of the sun will give 50bn year habitable lifetimes

However, for the planet Gliese 581d things get a little interesting: it has a habitable zone lifetime of around 50 billion years! That is more than 10 times the age of the Earth and almost 4 times longer than the age of the universe. This unbelievable timescale is due to a simple quirk of nature. While the brightest stars live fast and die young, some of the smallest stars can survive for hundreds of billions of years; dozens of times older than our sun will ever manage. What’s more these small stars evolve extremely slowly, allowing a well-placed planet to be habitable for much longer than planets in our solar system. If Earth could allow such a plethora of unique and complex species in only 4 billion years, imagine what could happen on an earth-like planet similar to Gliese 581d with 50 billion years of summer?

What all this goes to show is that we already know of places in the universe where life may be able to take hold and survive for billions of years. Some of these planets may be lifeless until long after the Earth is toast, only to warm up and spend 50 billion years in the planetary sweet spot. And even in our solar system life-friendly temperatures may have existed on Venus and may yet occur on Mars, springing new possibilities of life. As I’m sure you’ll agree; that’s a much better message to spread than ‘The Earth is Doomed’.

‘Earth Is Doomed’ articles:

PS: This was the first scientific paper ever to be published with my name on. To be able to write “myself and colleagues at the UEA came up with in a paper published in Astrobiology” and to say my handiwork is currently being studied by readers of dozens of news outlets makes me as giddy as a small child on christmas.

PPS: My contribution to the paper was to take complex models of how all stars evolve and produce a mathematical function allowing the luminosity for any time period and any stellar mass to be immediately calculated. This is the first step to working out how the habitable zone migrates and hence the habitable lifetime of any planet sat in it’s path. The majority of the work was performed by Andrew Rushby (who wrote a similar blog today) and Mark Claire, both of whom I am incredibly grateful to for the chance to be involved in this work.