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 that 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 19th century 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]. This method can now be used to determine what other planets contain if their starlight passes through their atmosphere^[3b].

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.

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

Here, v_radial 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.

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.

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,

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

Here,

P is the planet's period, determined using the Doppler equation. 2πr is the circumference of a circle of radius r. In this case, r is the distance between the star and the planet. r can be found using Kepler's 3rd law. This can be derived using the same assumptions from:

Now using

gives

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

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 planet's density since:

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 (L_s). This is because the power (energy/second) absorbed by the planet from the starlight (P_in) is the same as the power emitted by the planet (P_out).

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:

Here σ is the Stefan–Boltzmann constant and 4πR_p² = the surface area of a sphere. This is the area the power is emitted over, where R_p 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 (L_s/4πr²), multiplied by the ratio that's absorbed by the planet (1 - a), multiplied by the area of the planet illuminated by the star (πR_p²).

These can be rearranged as

gives

The luminosity (L_s) 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].

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, can be created by life but 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.

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 years 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 because 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.