Spectroscopy and biosignatures: How we'll find evidence of life on other planets

The spectra of the Sun showing spectral lines.

Image credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF/Public domain.

First published on 22nd April 2017. Last updated on 5th June 2017 by Dr Helen Klus

1. What we can learn from telescopes

Scientists have found thousands of planets outside of the Solar System[1], many of which are thought to be habitable[2a]. The next step is to search for signs of life.

While this may be possible with current and upcoming telescopes, like the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST), NASA is currently considering two projects that will be able to directly image Earth-like planets. These will be able to detect oceans and continents, as well as biosignatures in the atmosphere[3a].

These are known as the Habitable Exoplanet Imaging Mission (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR).

We can learn a lot about other planets using telescopes. We can use the radial velocity method to measure the velocity of stars, and from this we can work out the period of any planets and get a rough idea of their mass. We can then use the transit method to work out their radius[4].

The mass and radius can be used to determine the density of a planet, which gives us a good idea of what it's made of.

The temperature of a planet can be worked out from the star's luminosity and its albedo. This determines whether it contains liquid water on the surface. Planets that contain liquid water are referred to as habitable.

Almost everything else we can discover about other planets is done using spectroscopy.

1.1 The period and mass using the radial velocity method

The radial velocity method splits light from a star into a spectrum. This is known as spectrometry. The light spectra of stars have dark lines known as spectral or Fraunhofer lines. These occur when the starlight passes through clouds on the way to Earth.

Atoms in the clouds can absorb some of the photons in the light. These photons have specific energies, which depend on what the atom is made of. Different energies correspond to different wavelengths, and hence different colours, and so specific parts of the light's spectrum is missing.

These lines were discovered in the early 1800s by placing a prism in front of a telescope[5][6]. The lines were mapped by passing light in front of elements and molecules in the laboratory[7]. Now this method can be used to determine what other planets contain as their starlight passes through their atmosphere[3b].

Spectrum of the Sun showing main spectral lines.

The spectrum of the Sun showing missing wavelengths (K and H = Calcium, D = Sodium, E = Iron, C and F =Hydrogen, and A and B = Molecular Oxygen). The wavelengths are missing because sunlight passes through clouds made of elements and molecules, which absorb specific energies of light. Image credit: MaureenV/Phrood/Saperaud/Cepheiden/Public domain.

Plot of intensity against wavelength for sunlight.

A plot of the spectrum above where the dark lines can be seen in dips in intensity. Image credit: Remember the dot/Eric Bajart/CC-SA.

Spectral lines move if the light comes from a star that is moving. This is due to the Doppler effect.

The Doppler effect was discovered for sound and then light in the 1840s[8][9]. This shows that objects, like ambulance sirens, sound higher pitched if the sound is moving towards you. This is because the wavelength becomes lower.

When the wavelength of light becomes lower, the light looks bluer, and so when a star is moving towards us, the light looks bluer and the spectral line moves towards the blue end of the spectrum. When the star is moving away, the light looks redder and the spectral line moves towards the red end of the spectrum.

Animation demonstrating the Doppler effect.

The Doppler effect. Image credit: Charly Whisky/CC-SA.

Image of unshifted, redshifted, and blueshifted spectral lines. In redshifted spectra, the dark lines are closer to the red end of the spectrum, in blueshifted spectra, they are closer to the blue end.

Image credit: Cool Cosmos/Public domain.

The velocity of a star can be found by looking at how far the spectral line has moved, and in which direction. The Doppler equation for light shows that:

vradial/c
=
Δλ/λ
.

Here, vradial is the star's radial velocity, c is the speed of light, λ is the wavelength of the line on Earth, and Δλ is the difference between this and the wavelength from the star's light.

This velocity is known as the radial velocity because it only measures the velocity that the star moves towards us or away from us. This means that it's only completely accurate for stars that we see edge on.

Animation of a planet and a star orbiting a common centre of mass from the top.

A planet and a star orbiting a common centre of mass from the top, where there is no radial velocity. Image credit: Zhatt/Public domain.

Animation of a planet and a star orbiting a common centre of mass from the side.

A planet and a star orbiting a common centre of mass from the side, where the velocity is the radial velocity. Image credit: Reyk/Public domain.

We will know if a star is in orbit with something, another star or a planet for example, because then it will not just spin in a circle, it will orbit the common centre of mass of both objects. This causes it to ‘wobble'. We will know that we've seen a star edge on if the other star or a planet passes in front of it. This is known as a transit or an eclipse.

We can use the radial velocity method to determine the velocity of the star and the star's period. This is the same as the period of the other star or planet.

Pp = Ps

Here, P stands for period, p stands for planet and s for star.

Both objects will have the same period and the same momentum in order to remain stable.

Momentum = m × v, where m is mass and v is velocity, and so:

mp vp = ms vs,
mp =
msvs/vp
.

The velocity of the star is determined from the Doppler equation. There's a very strong connection between the mass of stars and their most prominent colour, which is shown in the H-R diagram, and so you can look up the star's mass.

You can estimate the velocity of the planet if you assume it's directly orbiting the star in a circle using:

Velocity =
Distance/Time
.

Here,

vp =
r/Pp
.

P is the planet's period, determined using the Doppler equation. r is the circumference of a circle of radius r. In this case, r is the distance between the star and planet. r can be found using Kepler's 3rd law. This can be derived using the same assumptions from:

Centripetal force = Gravitational force
mpvp2/r
=
Gmpms/r2
,
vp =
Gms/r
.

Now using:

r/Ps
=
Gms/r
.

Gives:

r3 =
GmsPs2/2
.

1.2 The radius using the transit method

The transit method can be used to determine a planet's radius if the radial velocity is known. This is done by measuring the dip in brightness as the planet passes in front of the star.

The brightness gets lower as the planet starts to pass in front of the star. It reaches a minimum when the whole of the planet is in front of the star and then begins to increase again as the planet leaves. The radius of the planet comes from timing how long it takes for the light to reach a constant value and using:

Velocity =
Distance/Time
.
Diagram showing how the total brightness of two stars in a binary system changes as one star passes in front of another.

The radius of objects that eclipse each other can be determined by measuring their brightness. Image credit: Helen Klus/CC-NC-SA.

1.3 The density and composition of a planet

We can estimate a planet's mass using the radial velocity method, and estimate its radius using the transit method. This means that we can work out the planets density since:

Density =
Mass/Volume
.

And the volume of a sphere = 4/3πr3.

If we know the density of a planet, this gives us a good idea of what it's made of. Gaseous planets, for example, are less dense than water worlds, which are less dense than rocky planets.

1.4 The temperature and albedo of a planet

The temperature of a planet can be determined from r, the planet's albedo (a), and the star's luminosity (Ls). This is because the power (energy/second) absorbed by the planet from the starlight (Pin) is the same as the power emitted by the planet (Pout).

Pout = Pin

The power that the planet emits is radiated out as blackbody radiation. This means that the relationship between power and temperature is given by the Stefan–Boltzmann law:

Pout = 4πRp2σTp4.

Here σ is the Stefan–Boltzmann constant and Rp2 = the surface area of a sphere. This is the area the power is emitted over where Rp is a radius, in this case, it's the radius of the planet.

The power in is the same as the flux - the power emitted by the star per unit area (Ls/4πr2), multiplied by the ratio that's absorbed by the planet (1 - a), multiplied by the area of the planet illuminated by the star (πRp2).

Pin =
πRp2Ls(1 - a)/r2
.
Diagram showing how light is emitted from a star covering a spherical area.

Image credit: Helen Klus/CC-NC-SA.

These can be rearranged so that:

Rp2σTp4 =
πRp2Ls(1 - a)/r2
,

gives:

Tp4=
Ls(1 - a)/16πr2σ
.

The luminosity (Ls) can be calculated by counting how many photons are emitted per second. The power that is actually absorbed by the planet depends on its albedo (a). Something like ice, which is very reflective and so very bright, has a high albedo, something like rock, which is very dark, has a low albedo.

The albedo can be calculated from the brightness, where the brightness of the planet can be worked out using the transit method.

A planet is said to be in the habitable zone if it's the right temperature to have liquid water on its surface. If it is colder, the water will freeze and if it's warmer, the water will boil away. The habitable zone is important to astronomers because planets with liquid water are thought to be more likely to contain life[10].

Painting showing ice on a planet orbiting a red dwarf star.

Artist's impression of TRAPPIST-1 f. Image credit: NASA/JPL-Caltech/Public domain.

2. Signs of life on other planets

Planets may be habitable if they contain liquid water. They might be even more likely to contain life if they have an atmosphere like Earth's. Carbon dioxide, for example, may be created by life, however it can also be created in other ways and so is not strong evidence for life on its own. Elements or molecules that are more likely to be created by life, and that can be found using spectroscopy, are known as biosignatures[3c][11].

Biosignatures include molecular oxygen (O2), ozone (O3), and nitrous oxide (N2O), which are produced by bacteria, methane (CH4), which is produced by archaea, and chloromethane (CH3Cl), which comes from seaweed.

Evidence of chlorophyll may also be found using spectroscopy. Chlorophyll is a molecule found in green plants, and if much of the surface of a planet is covered in green plants, light will reflect off them. Evidence of this may show up in the star's spectra if there is very little cloud cover and so light can hit the surface and then be reflected back out into space[12][13].

Evidence of intelligent life may be easier to find, assuming they do not intentionally hide their presence. Strange fluctuations of light, for example, may show evidence of a megastructure like the composition of a Dyson sphere[14]. Fluctuations can also be caused by natural objects, like comets, however intelligent life forms could also broadcast their presence. They could do this by using artificial objects to change the shape of their light curve or by transmitting information using lasers[15].

3. Future telescopes

We can currently learn about exoplanets using the Hubble Space Telescope (HST) and the Spitzer Space Telescope (SST). We will be able to get even better results with the James Webb Space Telescope (JWST), which is due to launch in 2018, and using the largest telescopes under construction on Earth such as the European Extremely Large Telescope (E-ELT). This is currently being constructed in Chile and may be operational by 2024.

NASA is currently considering two space telescopes that may be funded in 2020 for launch in the 2030s[3d]. These are the Habitable Exoplanet Imaging Mission (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR).

HabEx is designed specifically to take spectra and direct photographs of Earth-like planets in the habitable zone. LUVOIR would also spend time looking at other astronomical objects like black holes.

4. Where to look next; examples of possible habitable planets

Over 50 potentially habitable planets have been discovered so far[2b]. About 20 of these are about the size of the Earth, and about 30 are larger, known as super-Earths or Mini-Neptunes. These include: LHS 1140 b, Proxima Centauri b, and TRAPPIST-1 e.

The TRAPPIST-1 system compared to the Solar System.

The TRAPPIST-1 system. Image credit: NASA/JPL-Caltech/Public domain.

LHS 1140 b is about 39 light years away and is about 5 billion years old, about half a billion years older than the Earth. It's about 1.4 times the size of the Earth and orbits a red dwarf star. This means that any plants it contains may be red rather than green[16]. A year on LHS 1140b only lasts about 25 Earth-days and a day on LHS 1140b lasts about 130 Earth-days[17].

Proxima Centauri b is only about 4 light years away, orbiting the closest star to the Sun. It is about 4.9 billion year old and about 0.8-1.4 times the size of the Earth. It also orbits a red dwarf star but in this case, it is part of a triple star system with Sun-like star Alpha Centauri A and orange dwarf star Alpha Centauri B. A year on Proxima Centauri b is only about 11 Earth-days[18].

TRAPPIST-1 e is part of a system of seven planets that orbit a star about 40 light years away. Three of these planets may be habitable: TRAPPIST-1 e, f, and g. They all have about the same radius as the Earth and years of less than 13 Earth-days. They also orbit a red dwarf[19].

The fact that so many habitable planets have short years is due to the fact that planets are easier to find the closer they are to their star. To be this close and have liquid water on their surface, they must orbit a star that's not as hot as the Sun, like red dwarfs. Many more types of habitable planets are expected to be found with the next generation of telescopes.

5. References

  1. Exoplanets.org, 'The Exoplanet Data Explorer', last accessed 01-06-17.

  2. (a, b) The Planetary Habitability Laboratory, 'Habitable Exoplanets Catalog', last accessed 01-06-17.

  3. (a, b, c, d) Deming, L. D. and Seager, S., 2016, 'Illusion and Reality in the Atmospheres of Exoplanets', Journal of Geophysical Research: Planets, 122, pp.53–75.

  4. von Berlepsch, R., 2011, 'Reviews in Modern Astronomy, Deciphering the Universe through Spectroscopy', John Wiley & Sons.

  5. Wollaston, W. H., 1802, 'A method of examining refractive and dispersive powers, by prismatic reflection', Philosophical Transactions of the Royal Society of London, 92, pp.365-380.

  6. Fraunhofer, J., 1817, 'Determination of the Refractive and Dispersive Indices for Differing Types of Glass in Relation to the Perfection of Achromatic Telescopes', Denkschriften der Bayerischen Akademie der Wissenschaften, 5, pp.193-226.

  7. Kirchhoff, G. and Bunsen, R., 1860, 'Chemical Analysis by Observation of Spectra', Annalen der Physik und der Chemie, 110, pp.161-189.

  8. Doppler, C., 1842, 'Ueber das farbige Licht der Doppelsterne und einiger anderer gestirne des Himmels' ('On the coloured light of the binary stars and some other stars of the heavens'), Proceedings of the Royal Bohemian Society of Sciences, 2, pp.465-482.

  9. Gregersen, E., 2011, 'The Britannica Guide to Sound and Light', The Rosen Publishing Group.

  10. NASA, 'NASA Finds Earth-sized Planet Candidates in the Habitable Zone', last accessed 01-06-17.

  11. Seager, S., 2014, 'The future of spectroscopic life detection on exoplanets', Proceedings of the National Academy of Sciences, 111, pp.12634-12640.

  12. Brandt, T. D. and Spiegel, D. S., 2014, 'Prospects for detecting oxygen, water, and chlorophyll on an exo-Earth', Proceedings of the National Academy of Sciences, 111, pp.13278-13283.

  13. Patty, C. L., Visser, L. J., Ariese, F., Buma, W. J., Sparks, W. B., van Spanning, R. J., Röling, W. F., and Snik, F., 2017, 'Circular spectropolarimetric sensing of chiral photosystems in decaying leaves', Journal of Quantitative Spectroscopy and Radiative Transfer, 189, pp.303-311.

  14. Harp, G. R., Richards, J., Shostak, S., Tarter, J. C., Vakoch, D. A. and Munson, C., 2016, 'Radio SETI observations of the anomalous star KIC 8462852', The Astrophysical Journal, 825, pp.155.

  15. Kipping, D. M. and Teachey, A., 2016, 'A cloaking device for transiting planets', Monthly Notices of the Royal Astronomical Society, 459, pp.1233-1241.

  16. NASA, 'NASA Predicts Non-Green Plants on Other Planets', last accessed 01-06-17.

  17. Dittmann, J. A., et al, 2017, 'A temperate rocky super-Earth transiting a nearby cool star', Nature, 544, pp.333–336.

  18. Anglada-Escudé, G., et al, 2016, 'A terrestrial planet candidate in a temperate orbit around Proxima Centauri', Nature, 536, pp.437-440.

  19. Gillon, M., et al, 2017, 'Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1', Nature, 542, pp.456-460.

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