Extraterrestrial Life: The Giants are Coming…

by Paul Gilster on September 11, 2015

Finding a biological marker in the atmosphere of an exoplanet is a major goal, but as Ignas Snellen argues in the essay below, space-based missions are not the only way to proceed. A professor of astronomy at Leiden University in The Netherlands, Dr. Snellen makes a persuasive case that technologies like high dispersion spectroscopy and high contrast imaging are at their most effective when deployed at large observatories on the ground. A team of European observers he led has already used these techniques to determine the eight-hour rotation rate of Beta Pictoris b. We’ll need carefully conceived space missions to study those parts of the spectrum inaccessible from the ground, but these will find powerful synergies with the next generation of giant Earth telescopes planned for operations in the 2020s.

by Ignas Snellen


While I was deeply involved by my PhD project, studying the active centers of distant galaxies, a real scientific revolution was unfolding in a very different field of astronomy. In the mid-1990s the first planets were found to orbit stars other then our Sun. For several years I managed to ignore it. Not impeded by any knowledge I was happy to join the many skeptics to dismiss the early results. But soon they could be ignored no more. And when the first transiting planet was found and a little later its atmosphere detected, I radically changed research field and threw myself, like many others, on exoplanet research. More than a decade later the revolution is still going strong.


Not all scientific endeavors were successful during this twenty-year period. Starting soon after the first exoplanet discoveries, enormous efforts were put in the design (and getting the political support) for a spacecraft that could detect potential biomarker gases in the atmospheres of nearby planet systems. European astronomers were concentrating on DARWIN. This mission concept was composed of four to five free-flying spacecraft carrying out high-resolution imaging using nulling interferometry, where the starlight from the different telescopes is combined in such way that it cancels out on-axis light, leaving the potential off-axis planet-light intact. After a series of studies over more than a decade, in 2007 the European Space Agency stopped all DARWIN developments – it was too difficult. Over the same time period, several versions of the Terrestrial Planet Finder (TPF) were proposed to NASA, including a nulling interferometer and a coronagraph. The latter uses a smart optical design to strongly reduce the starlight while letting any planet light pass through. Also these projects have subsequently been cancelled. Arguably an even bigger anticlimax was the Space Interferometry Mission (SIM), which was to hunt for Earth-mass planets in the habitable zones of nearby stars using astrometry. After being postponed several times, it was finally cancelled in 2010.

How pessimistic should we be?

Enormous amounts of people’s time and energy were spent on these projects, costing hundreds of millions of dollars and euros. A real pity, considering all the other exciting projects that could have been funded instead. We should set more realistic goals and learn from greatly successful missions such as the NASA Kepler mission, which was conceived and developed during that same period. A key aspect of the adoption of Kepler as a NASA space mission was the demonstration of technological readiness through ground-based experiments (by Bill Borucki and friends). A mission gets approved only if it is thought to be a guaranteed success. It is this aspect that killed Darwin and TPF, and it is this aspect that worries me about new, very smart spacecraft concepts such as the large external occulter for the New World Mission. Maybe I am just not enough of a (Centauri) dreamer.

In any case, lead times of large space missions, as the Kepler story has shown, are huge. This implies that it is highly unlikely that within the next 25 years we will have a space mission that will look for biomarker gases in the atmospheres of Earth-like planets. If I am lucky I will still be alive to see it happen. My idea is – let’s start from the ground!

The ground-based challenge

The first evidence for extraterrestrial life will come from the detection of so-called biomarkers – absorption from gases that are only expected in an exoplanet atmosphere when produced by biological processes. The prime examples of such biomarkers are oxygen and ozone, as seen in the Earth’s atmosphere. Observing these gases in exoplanet atmospheres will not be the ultimate proof of extraterrestrial life, but it will be a first step. These observations require high-precision spectral photometry, which is very challenging to do from the ground. First of all, our atmosphere absorbs and scatters light. This is a particular problem for observations of Earth-like planets, because their spectra will show absorption bands at the same wavelengths as the Earth’s atmosphere. In addition, turbulence in our atmosphere causes the light that enters ground-based telescopes to become distorted. Therefore, light does not form perfect incoming wavefronts, hampering high-precision measurements. Furthermore, when objects are observed for a longer time during a night, their light-path through the Earth atmosphere changes, as does the way starlight enters an instrument, making stability a big issue. These are the main reasons why many exoplanet enthusiasts thought that it would be impossible to ever probe exoplanet atmospheres from the ground.

The technique

Work over the last decade has shown that one particular ground-based technique – high dispersion spectroscopy (HDS) – is very suitable for detecting absorption features in exoplanet atmospheres. The dispersion of a spectrograph is a measure of the ‘spreading’ of different wavelengths into a spectrum of the celestial object. Space telescopes, such as the Hubble Space Telescope (HST), Spitzer, and the future James Webb (JWST) have instruments on board that are capable of low to medium dispersion spectroscopy, where the incoming light can be measured at typically 1/100th to 1/1000th of a wavelength. With HDS, precisions of 1/100,000th of a wavelength are reached – hence about two orders of magnitude higher than from space. For two reasons this can practically only be done from the ground: 1) the physical size of a spectrograph scales with its dispersion, meaning that HDS instruments are generally too big to launch to space. 2) At high dispersion the light is spread very thinly, requiring a lot of photons to do it right, hence a large telescope. For example, the hot Jupiter tau Bootis b required 3 nights on the 8m Very Large Telescope to measure carbon monoxide in its atmosphere. Scaling this to the HST (pretending it would have an HDS instrument) it would have cost on the order of 200 hours of observing time – more than was spent on the Hubble Deep Field. Hence, HDS is the sole domain of ground-based telescopes.

The high dispersion is key to overcome the challenges that arise from observing through the Earth’s atmosphere. At a dispersion of 1/100,000th of a wavelength, HDS measurements are sensitive to Doppler effects due to the orbital motion of the planet. E.g. the Earth moves with nearly 30 km/sec around the Sun, while hot Jupiters have velocities of 150 km/sec or more. This means that during an observation, the radial component of the orbital velocity of a planet can change by tens of km/sec. While this makes absorption features from the planet move in wavelength, any Earth-atmospheric and stellar absorption lines remain stationary. Clever data analysis techniques can filter out all the stationary components of a time-sequence of spectra, while the moving planet signal is preserved. Ultimately, the signal from numerous individual planet lines can be added up together to boost the planet signal using the cross-correlation technique – weighing the contribution from each line by its expected strength.

Screenshot from 2015-09-11 08:59:46

Image: Illustration of the HDS technique, with the moving planet lines in purple.

So why does this work? Although the Earth atmosphere has a profound influence on the observed spectrum, the absorption and scattering processes are well behaved on scales of 1/100,000th of a wavelength and can be calibrated out. The signal of the planet can be preserved, even if variations in the Earth atmospheres are many orders of magnitude larger. In this way starlight reflected off a planet’s atmosphere can be probed, but also a planet’s transmission spectrum – when a planet crosses the face of a star and starlight filters through its atmosphere. In addition, a planet’s direct thermal emission spectrum can be observed. This is particularly powerful in the infrared. And it works well! In the optical, absorption from sodium has been found in the transmission spectra of several exoplanets. In the near-infrared, carbon monoxide and water vapor have been seen in both the transmission spectra as well as thermal emission spectra of several hot Jupiters – on par with the best observations from space. In the next two years new instruments will come online (such as CRIRES+ and ESPRESSO on the VLT) that will take this significantly further – allowing a complete inventory of the spectroscopically active molecules in the upper atmospheres of hot Jupiters, and extending this research to significantly cooler and smaller planets.

One step beyond

There is more. The HDS technique makes no attempt to spatially separate the planet light from that of the much brighter star – it is only filtered out using its spectral features. Hot Jupiters are much too close to their parent stars to be able to see them separately anyway. However, planets in wider orbits can also be directly imaged, using high-contrast imaging (HCI) techniques (also in combination with coronography). This technique is really starting to flourish using modern adaptive optics in which atmospheric turbulence is compensated by fast-moving deformable mirrors. A few dozen planets have already been discovered using HCI, and new imagers like SPHERE on the VLT and GPI on Gemini, which came online last year, hold a great promise. What I am very excited about is that HDS combined with HCI (let’s call it HDS+HCI) can be even more powerful. While HDS is completely dominated by noise from the host star, HCI strongly reduces the starlight at the planet position – increasing the sensitivity of the spectral separation technique used by HDS by orders of magnitude. Last year we showed the power of HDS+HCI by for the first time measuring the spin velocity of an extrasolar planet, showing beta Pictoris b to have a length of day of 8 hours. [For more on this work, see Night and Day on β Pictoris b].


Image: HDS+HCI observations of beta Pictoris b.

The giants are coming

Both the US and Europe are building a new generation of telescopes that can truly be called giants. The Giant Magellan Telescope (GMT) will consist of six 8.4m mirrors, equivalent of one 24.5m diameter telescope. The Thirty Meter Telescope (TMT) will be as large as the name suggests, while the European Extremely Large Telescope (E-ELT) will be the largest with an effective diameter of 39m. All three projects are in a race with each other and hope to be fully operational in the mid-2020s.

Size is everything in this game – in particular for HDS and HDS+HCI observations. HDS benefits from the number of photons that can be collected, which scales with the diameter squared. Taking into account also other effects, the E-ELT will be >100 times faster than the VLT (in particular using the first-light instrument METIS, and HIRES). This will bring us near the range needed to target molecular oxygen in the atmospheres of Earth-like planets that transit nearby red dwarf stars. We have to be somewhat lucky for such nearby transiting systems to exist, but simulations show that the smaller host star makes the transmission signal of molecular oxygen from an Earth-size planet similar to the carbon monoxide signals we already have detected in hot Jupiter atmospheres – it is just that the systems will be much fainter than tau Bootis requiring the significantly bigger telescopes. The technology is already here, but it is all about collecting enough photons. This could also be solved in a different way if even the ELTs turn out not to be large enough. HDS observations of bright stars do not require precisely shaped mirrors and this could be achieved by arrays of low-precision light collectors, but this is something for the more distant future.


Image: Artist impression of the E-ELT – ready in 2024! (credit: ESO).

Even more promising are the high-contrast imaging capabilities of the future ELTs. Bigger telescopes not only collect more photons, but also see sharper. This makes their capability to see faint planets in the glare of bright stars scale with telescope size up to the fifth power, making the E-ELT more than a 1000 times faster than the VLT. Excitingly, rocky planets in the habitable zones of nearby planets become within reach. Again, simulations show that their thermal emission can be detected around the nearest stars, while HDS+HCI at optical wavelengths can target their reflectance spectra, possibly even including molecular oxygen signatures.

Realistic space missions

Whatever happens with space-based exoplanet astronomy, ground-based telescopes will push their way forward towards characterizing Earth-like planets. This does not mean there is no need for space missions. First of all, I have not done justice to the fantastic, groundbreaking exoplanet science the JWST is going to provide. Secondly, a series of transit missions, TESS from NASA (launch 2017), and CHEOPS and PLATO from ESA (Launch 2018 & 2024), will discover all nearby transiting planet systems, a crucial prerequisite for much of the science discussed here.

Above all, ground-based measurement will not be able to provide a complete picture of a planet’s atmosphere – simply because large parts of the planet’s spectrum are not accessible from the ground. This will mean that the ultimate proof for extraterrestrial life will likely have to come from a space mission type DARWIN or TPF. Imagine how a ground-based detection of say water in an Earth-like atmosphere would open up political possibilities, but the right timing for such missions is of upmost importance. Aiming too high and too early means that lots of time and money will be wasted, at the expense of progress in exoplanet science. It is good to dream, but we should not forget to stay realistic.

Further reading

Snellen et al. (2013) Astrophysical Journal 764, 182: Finding Extraterrestrial Life Using Ground-based High-dispersion Spectroscopy (http://xxx.lanl.gov/abs/1302.3251).

Snellen et al. (2014), Nature 509, 63: Fast spin of the young extrasolar planet beta Pictoris b (http://xxx.lanl.gov/abs/1404.7506).

Snellen et al. (2015), Astronomy & Astrophysics 576, 59: Combining high-dispersion spectroscopy with high contrast imaging: Probing rocky planets around our nearest neighbors (http://xxx.lanl.gov/abs/1503.01136).


Tom Mazanec September 11, 2015 at 12:53

What is the prospect for the Colossus telescope:
which is, unlike the OWL, not defunct through overreaching at this time?

ljk September 11, 2015 at 13:55

They should never have put the word Overwhelmingly in the name. Seriously.

spaceman September 11, 2015 at 13:57

I used to think that the detection of biomarkers in the atmosphere of an exoplanet, say, 50 light years away would count as definitive evidence of a second origin of life in the Universe. However, recent discussions of panspermia bubbles seem to put a kink in this idea. How far away would a world with detected biomarkers have to be in order for us to be statistically very sure that it doesn’t share a common origin with Earth based biochemistry?

Alex Tolley September 11, 2015 at 14:22

Fascinating article. It is encouraging that Earth telescopes may be able to detect good biosignatures within such a short time frame. We may get some answers to plug into the Drake equation for that variable relatively soon and inexpensively. To know whether Earth type life on the surface of a planet is extremely rare or common is exciting and can direct our thoughts about interstellar probes and even travel.

It also promises to obsolete all those SF stories where travelers head off into the unknown. Long before then, we should be able to characterize the planets of reachable stars.

Andrew Palfreyman September 11, 2015 at 16:45

Thank you for a superb article. It really captures the excitement and the near-term possibilities. We live in interesting times!

Joe September 11, 2015 at 18:16

Earth based science is almost always better than space based science. Normal gravity, easier to repair, cheaper to build, accessible to tourists, more jobs created, especially lower skilled jobs like site preparation and building construction. These things matter a lot to Congress. The potential of earth based telescopes could really re-energize the space program, even if we don’t actually go into space.

Joy September 11, 2015 at 18:59

Thank you Ignas,
We are all dreamers here to various degrees, but in the end our dreams must have some relationship with the real world. Telescope generations tend to be about 30 years. We can see the gap between Mt Wilson 1917 and Palomar 1948 which had a great depression and a world war in between. Likewise Keck 1993 and the new giant ground based telescopes 202x? will be about 30 years. In space, it is no different, as Hubble 1990 will finally be followed by Webb no sooner than 2018.

It is nice to dream about the 3rd generation giant space telescope, but that is unlikely to fly before mid century, by which time many of us dreamers will be dead. Furthermore, there are a multitude of reasons to believe that the coming 30 years post the launch of Webb will be even more chaotic and eventful than the 30 historical years post Mt Wilson. The giant ground based light buckets coming in the 2020s with the extraordinary detectors designed by brilliant optical and IT engineers will deliver the best data most of us will ever live to see. We will be most grateful for it.

“Ah, but a man’s reach should exceed his grasp, Or what’s a heaven for?”
– Robert Browning

ReaperX September 11, 2015 at 20:58

A detail. You say that a telescope’s ability to see faint planets in the glare of bright stars scale with up to the 5th power of the diameter.

I know that the signal to noise ratio for imaging an unresolved point like sources goes up with the 4th power of D. Increased light gathering power contributes a factor D^2, and and due to increased linear resolution, the same energy is spread out over an area D^2 smaller, so that’s D^4.

I can see how the increased linear resolution would reduces glare from the star, but I wouldn’t expect that to scale with D beyond a certain sufficient resolution. Is it D^5 for “small” diameters and then D^4 after that?

Harold Daughety September 12, 2015 at 0:42

@Joy, I do not think I have ever seen so apt and application of the quote from Browning. It actually is the only Browning quote I have used: I prefer the poetry of Wordsworth and Tennyson but they write more often in metaphors rather than similes, so there are few opportunities for a one line zinger. And I have to agree the rest 0f the post. Well said!

ole burde September 12, 2015 at 6:57

In the best of worlds , the new generations of telescopes will deliver a ‘perfect’ target planet for exploration , just in time when the propulsion technology is ready for a serious injection of money . The ‘perfect’ target does not have to include biomarkers , even if this could be the easiest way to get funding for an interstellar probe . If no biomarkers where found in countless planets, where they could have been expected to exist , this in itself could become the basis for another driver of public opinion : a life- empty universe could make the case for seeding life on far away planets morally convincing , for segments of the public opinion that has never before taken an interest in our dream …So in both cases it will be a major asset to prove or disprove the exsistence of biomarkers .

G. Cecil September 12, 2015 at 7:59

Interesting summary, thank you. To complement, the AURA commissioned report on the 11m+ optical-UV-NIR High Definition Space Telescope (HDST) ‘astrophysics by exoplanet stealth’ is now at http://www.hdstvision.org/report/
A folded primary mirror is not the only option under study, but it pragmatically would fit on a cost-curve collapsing Falcon Heavy. The report likely underestimates progress possible on bio-signature detection from ground w/ specialized instruments on ELTs before this thing might fly in the mid-2030s. (The UV advantage of space remains for astrophysics.)

R Mark Elowitz September 12, 2015 at 9:25

It is of interest to note that the detection of O2 and CO2 in a planetary (or exomoon) atmosphere does not necessarily indicate the presence of life. These gases gas be produced by various abiotic sources. The recent discovery of a tenuous O2-CO2 atmosphere surrounding Saturn’s moon Rhea shows that an atmosphere comprised of these gases can be the result of abiotic processes.

Mike September 12, 2015 at 13:29

Great article.

exoplanetsRbust September 12, 2015 at 15:40

This is an interesting technique, but it’s hard to see it being applied to any targets other than hot or very young Jupiters. The targets done so far are very bright and easy to see : only 10 magnitudes fainter than their host star, with the planet itself being 15th magnitude or so. A reflected light Jupiter in a G star HZ would be at least 6 mag fainter than this, requiring a telescope with 15x larger diameter to collect the same signal. 15x larger than the 8m VLT is a 120m telescope, so even the E-ELT won’t be doing this as a combined light measurement.

In high contrast imaging the signal does not improve by the telescope diameter to the 4th power – the latter is only true for a uniform background. Instead the background in this case is residual starlight speckles, which current AO systems only control to a contrast level of 12 magnitudes or so. Future systems may improve on this by a few magnitudes; having a bigger telescope doesn’t actually help very much, as it’s the performance of the AO system itself that determines the contrast floor. A HZ Earth is 25 mag fainter than its parent G star, and 20 mag fainter than a parent M star. So there is a considerable gap between where ground AO is likely to be in the era of the E-ELT, and the contrast needed to image habitable planets.

Ground telescopes have done great things in exoplanet research and will continue to do so.
We should get everything out of them that we possibly can, as they will almost always be more affordable than space projects. But some things can only be done from space. Spectroscopy of habitable terrestrial planets is one of them, and is compelling enough to be worth the high cost.

Ignas September 12, 2015 at 15:40

@ReaperX: indeed, far enough away from the star it goes like D^4 (in the background limited regime). Close to the star its more like D^5.

Ignas September 13, 2015 at 12:19

@ exoplanetsRbust: Thanks for your comments. Note that the beta Pictoris b measurement was done in less than 1 hour over a smallish wavelength range. With CRIRES+ we can do it in less than 10 minutes. This is very easy, and not the limit of what the VLT can do. The 3rd paper reference goes into detailed simulations for the E-ELT, showing what you can do with Alpha Cen or Proxima (you can reach rocky planets). The HDS+HCI technique actually very nicely deals with speckles. The speckles have the star’s spectrum (and not that of the planet), and can therefore be filtered out – in the same way as done for HDS observations.

I agree with you that it all becomes much harder for stars at larger distances. This is one of the drawbacks of direct imaging – and much less so for the transit or radial velocity technique.

Astra September 15, 2015 at 14:12

One advantage of ground-based transit observations that has yet to be capitalized on IMO is the ability to conduct repeat observations to beat down systematics. Our two UV transit observations with HST/COS only allowed for 1-2 transits observed per target because observing time is so highly oversubscribed. On the ground, observers will have more flexibility to repeat the transit observations to separate repeatable features from secular variability. So far, I’ve been disappointed on how little this has been done but I think the field is finally maturing past the initial exciting but perhaps over-credulous phase to being more careful in the treatment of uncertainties.

Mike Hernitz September 25, 2015 at 0:12

Great article. Very encouraging to see progress in the field of exo-planet characterization using the spectroscopic techniques. As was pointed out, all else equal (when it comes to telescopes) bigger is better. I would like to see a very large aperture (at least 10 meters, but really as big as we can build it with present day techniques) telescope in space. Don’t need to bother with adaptive optics to unscramble atmospheric image blurring. See the whole sky. Operate 24/7/365. Oh yea, there are wavelengths that are so attenuated by our atmosphere that in a practical sense, you can’t see them from the ground. ASSEMBLE THE TELESCOPE IN SPACE USING METHODS LEARNED BUILDING THE SPACE STATION! Then move it to Sun – Earth L2. (Also, plan to service it with astronauts at L2 – too big an investment to just let it die if something breaks…) From L2 we could use a far field occulting disk to reduce glare from the parent star. (Probably can’t do that from the ground…) I’ve heard Space-X will be giving rides on their Falcon 9 heavy for around $100,000,000. This should keep the launch cost part of the budget for the big ‘scope in the pretty reasonable zone.

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