The existence of planets around other stars is now commonplace — over 5,000 exoplanets have been detected in 2023 compared to less than 50 known exoplanets in 2000. That’s a 100-fold increase in just over 20 years! The solar systems that have been found look very different from our own. Some have gas giants orbiting very close to their host stars and others have planets with sizes that are not seen amongst our eight planets. Let’s take a look at the diversity of planets and planetary systems that have been found and highlight a few of the most interesting, with respect to the possibility of finding life. We’ll conclude by asking how we might detect the presence of life on exoplanets.

Exoplanet Statistics

Now that a large sample of exoplanets have been detected, we can take stock of the types of planets that have been found. Overall, the transit method and radial velocity methods have yielded the most exoplanets to date, with 75% of known exoplanets found with the transit method and 19% with the radial velocity method. The animation below summarizes the history of planetary discovery. Note the explosion of detections with the transit method in 2014 when data from the Kepler mission started coming in.

Figures 1 and 2 below summarize the detections by the plotting the planet’s mass (Fig. 1) and the planet’s radius (Fig. 2) against how long it takes the planet to orbit its host star (its period). Each planet has a symbol to indicate which detection method was used to discover the exoplanet. We will focus on the radial velocity and transit methods discussed in the previous chapter, but some planets have been found with gravitational microlensing. The extended version of this chapter briefly describes timing variations (which enable the determination of the mass of the planet) and orbital brightness modulations.

There a few important limitations to Figures 1 and 2 that we need to note. Figure 1 shows the mass and orbital period of the exoplanets. The mass of an exoplanet can be found with the radial velocity (RV) method and the timing variations method. The transit method allows you to determine the radius of an exoplanet but not the mass. Not all exoplanets found with the transit method have been followed up with the RV method to get the mass, so not all planets found with the transit method are included in Figure 1.

In a similar way, not all exoplanets detected with the RV method are shown in Figure 2, and this is clear to see as Figure 2 is dominated by green squares for the transit method. Of the 1000+ stars found with the RV method, only the 66 that have had their radii determined are included in Figure 2.

In Figure 2, there is a gap for planets with radii of 4 to 10 times the radius of the Earth (4 R_Earth < R < 10 R_Earth), especially for planets that are orbiting close to their host stars. There are Jupiter-sized planets found orbiting close to their stars (hot Jupiters) as well as Earth-sized planets orbiting close — where are the planets with radii 4-10 R_Earth? Is this gap due to limitations of the detection methods or is this telling us something more general about the types of planets that form around stars? Since the transit method can detect stars below and above this range of radii, this suggests the gap (sometimes called the “hot Neptune desert”) is real and that it has something to do either with the formation or longevity of planets this size. One idea is that planets of this size are most susceptible to having their atmospheres completely evaporate away, making them planets that can transition in size and eventually have a smaller radius (where planets are found).

The types of planets being found are commonly divided into four main categories: gas giants, Neptune-like, super-Earths, and terrestrial. Figure 3 summarizes the fraction of each of these types that has been found. Super-Earths have radii between 1.2 and 2.8 that of Earth, and the other large group with sizes between 2.8 and 4 that of Earth are often called Neptune-like or mini-Neptunes. While the totals are split into about one-third each for gas giants, super-Earths, and Neptune-like exoplanets, we need to keep in mind that the transit method, which has found 75% of all exoplanets, is biased against very small, terrestrial-size planets. The smallest radius for an exoplanet detected with the transit method is about 30% the size of Earth’s radius (R = 0.30 R_Earth), so we cannot immediately conclude that there are fewer terrestrial exoplanets out there compared to the other types. In our own solar system, two of the eight planets, Mercury and Mars, have radii of 0.39 R_Earth and 0.53 R_Earth.

For a clearer look into the distribution of exoplanets by their size, Figure 4 shows a histogram of the radii for known transiting exoplanets that have a measured radius. This does not eliminate the bias against terrestrial planets but some interesting trends emerge.

First, we see a preponderance of exoplanets with sizes intermediate between the Earth and Neptune — the super-Earths and mini-Neptunes. No planets of these types exist in our solar system! What a remarkable discovery it is that the most common types of planets in the Galaxy are completely absent from our solar system and were unknown until the Kepler mission. Further, this idea that planets of roughly Earth’s size are so numerous is surely one of the most important discoveries of modern astronomy.

The TRAPPIST-1 system

One of the most intriguing planetary systems detected so far is TRAPPIST-1. The TRAPPIST-1 planetary system is about 40 light years away from Earth and has seven known exoplanets. The TRAPPIST-South survey program came online in 2010 by European astronomers using a small, ground-based telescope at La Silla Observatory in Chile to search for exoplanets with the transit method. The acronym TRAPPIST stands for “Transiting Planets and Planetesimals Small Telescope,” and is a nod to the popular Trappist beers found in Belgium.

In 2015, three planets were detected with this survey around a star that the team renamed TRAPPIST-1 (catchier than the original catalog name for this star, 2MASS J23062928-0502285). TRAPPIST-1 is a small, cool, red dwarf; the advantage of looking at small stars is that the transit signals from smaller, terrestrial planets are stronger. In 2017, four more planets were discovered around TRAPPIST-1, with the transit timing variations, bringing the total to seven planets orbiting TRAPPIST-1. All of these planets orbit at a distance that is smaller than the distance from the Sun to Mercury in our solar system and are similar in size to the rocky terrestrial worlds. Figure 5 below summarizes the properties of the seven known planets orbiting TRAPPIST-1 compared to our solar system’s four terrestrial planets.

Three of the seven planets orbiting TRAPPIST-1 are in the habitable zone — the distance from TRAPPIST-1 where the temperature is just right for liquid water. If there are three habitable worlds around the TRAPPIST-1 star and if technological civilizations evolve on any of these planets, there will be a powerful incentive to build spaceships that travel to these neighboring worlds. All of the TRAPPIST-1 detected to date are either terrestrial or super-Earths. We know this based on both the mass and radii of the TRAPPIST-1 planets, which together allow us to find their densities. In Figure 5, we see that most of the TRAPPIST-1 planets have densities close to Earth, with TRAPPIST-1 d having a lower density at 62% the density of Earth (ρ_T1d = 0.62 ρ_Earth).

The orbits of the seven TRAPPIST-1 planets are close to resonance, meaning that the ratio of the orbital periods are close to integer values. Planet e is in a 6-day orbit and planet g is in a 12-day orbit; planet f is in a 9.2-day orbit and planet h is in an 18.7-day orbit.

TRAPPIST-1 System with JWST

Since its discovery, the TRAPPIST-1 system has been followed up with several different telescopes, including Spitzer, Kepler, Hubble and the James Webb Space Telescope. Hubble collected spectra for all seven exoplanetary transits in the TRAPPIST-1 system to study the compositions of any atmospheres that these planets may hold.

The mid-Infrared Instrument (MIRI) on JWST was used to find the temperature of TRAPPIST-1 b by measuring the brightness of TRAPPIST-1 when planet b was behind the star (this is called a secondary transit or an occultation). The temperature of TRAPPIST-1 b was found to be about 500 K and further analysis showed that it is unlikely that this exoplanet has an atmosphere (read more at https://exoplanets.nasa.gov/news/1756/webb-measures-the-temperature-of-a-trappist-1-exoplanet/). The data was fit to models for a planet with an atmosphere and without, and the observations were a better match to the model for a planet that is bare rock (with no atmosphere).

The NIRISS (Near-Infrared Imager and Slitless Spectrograph) instrument aboard JWST was also used to study TRAPPIST-1 b. The NIRISS results confirmed that there is little evidence that TRAPPIST-1 b has an atmosphere — although it still cannot be ruled out — but also found that contamination from the star TRAPPIST-1 can compromise the analysis of spectra^[1]. M dwarfs like TRAPPIST-1 experience frequent flares of X-rays and other surface phenomenon, such as star spots and faculae, both of which are caused by magnetic fields. If a flare occurs during the measurement of the planet’s spectrum, this can create “ghost lines” that could be misinterpreted as an actual chemical signature in the spectrum. This study underscores the importance of taking into account the effects of outbursts of radiation frm the host star, especially M dwarfs, which are the most plentiful type of star in our Galaxy.

Assessing Exoplanetary Habitability

With some basic properties of an exoplanet — its mass, radius, semi-major axis and orbital period — and its host star, a critical assessment can be made as to whether the exoplanet is habitable. In this section, we look at some of the key questions to ask in carrying out such an assessment.

Density

The density of a planet is important in understanding its composition and surface: this tells us if the world is a gas giant, a rocky planet or something in between. To find the density of an exoplanet, both its radius and mass are required. Some examples are shown above on how to carry out density calculations for exoplanets.

A good resource for finding the mass and radius of an exoplanet, reported relative to Earth or Jupiter, is NASA’s Eyes on Exoplanets site: https://exoplanets.nasa.gov/eyes-on-exoplanets/

Once you know the approximate density of a planet (we say approximate here as the exact mass is usually not known, but instead the minimum mass found through the Doppler technique), you can then assess the surface properties by comparing to some benchmarks, such as the density of Earth and the density of Jupiter. Figure 7 below shows the densities for the planets in our solar system compared to those of exoplanets detected.

Surface Gravity

The surface gravity of a planet can give insight as to whether the world can hold onto an atmosphere. Without an atmosphere, life on any other world seems improbable. On Earth, the atmosphere keeps the surface temperature (relatively) stable and enables the Greenhouse effect; without our atmosphere, Earth would be a frozen ice planet.

The surface gravity of a world depends on its mass and radius as:

$$g = G\frac{M}{R^2}$$

An example of how to find the surface gravity of a world is shown here in the chapter on Laws of Motion and Gravity.

How much gravity at the surface is enough to retain an atmosphere? Atmospheric retention depends not only on the surface gravity but also the temperature. Gas molecules move faster in hot environments compared to cold ones.

Habitable Zone (liquid water?)

A key question to ask is whether or not the planet is in the habitable zone around its host star. The distance from the star to the edges of the habitable zone depend entirely on the parameters of the host star (and can be estimated using just the luminosity of the star, as shown in this example in the Habitable Zone chapter). With the inner and outer boundaries (that is, the distance from the star to the closest part of the HZ and the distance to the furthest part), the distance from the exoplanet to the star (its semi-major axis, a) can be compared to see if it falls within these boundaries. An example is shown in the box below.

Worked Example: Is a planet in the HZ around its star?

Using the luminosity of its host star, estimate whether the rocky exoplanet Kepler 452 b is in the habitable one around its host star.

First, a quick search* reveals that the luminosity of Kepler 452 (the star around which Kepler 452 b orbits) is about 1.2 times the luminosity of the Sun: L_K452b = 1.2 L_Sun

Now, we can use the relationships for the inner/outer bounds of the conservative HZ:

[latex]d_{inner,star} = 0.95 \sqrt{L_{star}/L_{Sun}}[/latex] AU

[latex]d_{outer,star}= 1.37 \sqrt{L_{star}/L_{Sun}}[/latex] AU

Since L_K452b = 1.2 L_Sun,

[latex]d_{inner} = 0.95 \sqrt{1.2}[/latex] AU = 1.04 AU

[latex]d_{outer} = 1.37 \sqrt{1.2}[/latex] AU = 1.50 AU

Next, let’s compare these bounds with the average distance of Kepler 452 b from Kepler 452:

$a$ = 1.05 AU

Kepler 452 d is very close to the inner edge of the HZ around Kepler 452. Yes — it is in the habitable zone.

Note that Kepler 452 has nearly the same luminosity as the Sun and the planet Kepler 452 is orbiting at about the same distance from Kepler 452 as the Earth does from the Sun. This is an intriguing example of an Earth-sized planet orbiting a Sun-like star…could Kepler 452 b have water on its surface?

*For a more precise estimate of the luminosity, look up the star on NASA’s exoplanet archive. You can also estimate a star’s luminosity relative to our Sun by using the Stefan-Boltzmann law, if you know its radius and temperature.

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For a graphic image, you can use the HZ generator tool at http://astro.twam.info/hz/

Try it! Generate the HZ around TRAPPIST-1.

To start, open the HZ sim link and click on the “Multiple” tab. Remove Star B (by clicking on the red X next to it). Now you can input the values for TRAPPIST-1 — note the units! You will need to adjust both the Plot Radius and the Resolution for the figure.

Go Deeper

Look back at the distances for each of the seven detected in exoplanets in the TRAPPIST-1 system. Which of these planets fall into the HZ around TRAPPIST-1, as shown in the figure above?

[Ans: c, d, e]

Tidal Locking

A world is tidally locked if the same side of it always faces the object that it orbits. The Moon, for example, is tidally locked to the Earth and we always see the same side of the Moon from Earth. It takes the Moon the same amount of time to make one rotation on its axis as it does to make one orbit around the Earth. Pluto and its moon Charon are also tidally locked.

Planets can be tidally locked to their host stars as well. This is a gravitational effect and depends on the mass of the star and the planet’s distance to the star. Over time, the rotation (spin) of the planet is gravitationally synchronized with the orbit. Figure 8 below summarizes where the “tidal lock radius” line (to the left of this line a planet is tidally locked) for stars by their spectral type and the distance that a planet is from the star. Our solar system is shown for comparison. Mercury is shown to the left of the tidal lock radius line, and Mercury is in a 3:2 orbital resonance with the Sun (because it also experiences gravitational pulls from Venus, Mercury is not in a 1:1 orbital resonance, which is the case for tidal locking).

Biosignatures

How might we be able to tell if there is actually life on an exoplanet? Scientists search for signs of life by looking for telltale signatures.

It is apt to begin a discussion of biosignatures with the adage “Extraordinary claims require extraordinary evidence.” As we look at some possible signatures of life, we cannot immediately conclude that the detection of just one biosignature means we have found other life in the universe. For any potential biosignature detected, absolutely all abiotic sources must be ruled out. False positives can be difficult to identify but an exceedingly critical analysis is in order.

A good place to start in pondering atmospheric biosignatures on other planets is to see what Earth’s atmosphere looks like — and what another civilization would see if they took a spectrum of the Earth. Figure 9 shows the spectrum of Earth’s atmosphere at optical and near-IR wavelengths. The first thing that you may notice is water…and recall “Follow the water!”. This is a sign of liquid water oceans on Earth, so we are indeed excited when H₂O is detected in an exoplanetary atmosphere, but water vapor does not necessarily mean that there is water on the surface (Jupiter and Saturn have water vapor in their atmospheres).

Carbon dioxide is also a byproduct of life on Earth — all animals exhale CO₂ and CO₂ is released when plants decay. But CO₂ can also be produced though volcanic eruptions, and its concentration can be changed with anthropogenic (man-made) sources such as the burning of fossil fuels or deforestation. To further exemplify this point, Figure 10 shows the atmospheric profiles for Venus, Earth and Mars.

Mars, Earth and Venus all have CO₂ in their atmospheres but only Earth shows water and ozone (which is related to O₂). Let’s take a deeper look at some potential biosignatures and their abiotic sources. As we’ll see, oxygen alone or methane alone would not be a good biosignature, but the combination of oxygen and methane is a smoking gun for life. At the temperatures and pressures in the atmosphere of Earth, oxygen would react quickly with methane, producing carbon dioxide and water. The simultaneous existence of these elements on Earth occurs only because life is producing these elements and maintaining chemical disequilibrium.

Oxygen

Life on Earth keeps O₂ in our atmosphere — it is a byproduct of life on Earth. Ozone — O₃ — is formed when an O₂ molecule is broken apart and one of the single oxygen atoms combines with another O₂ molecule. So, the presence of ozone is a proxy for O₂. Of course, just O₃ is not enough and a more compelling case would be made if both O₃ and another biosignature were detected. For example, ozone and water in our atmosphere point to habitable conditions on the surface.

False negatives need to also be considered in addition to false positives. Just because an atmosphere does not show O₂, this does not necessarily mean that there is no life. On Earth, cyanobacteria began putting O₂ into our atmosphere perhaps as early as 3.8 billion years ago (stromatolites) but it would not have showed up in a spectrum because it would immediately react with other elements. On a similar note, if there was a very low level of O₂, our instruments may not be sensitive enough to detect it. Finding unambiguous signs of life is a tricky game to play.

Methane

Methane (CH₄) is another molecule that can be produced both biotic and abiotic sources. Methane is a natural byproduct of volcanic outgassing and, in our own solar system, the lifeless worlds Uranus and Neptune have methane in their lifeless atmospheres. Methane can also be produced geologically in hydrothermal vent communities through a process called serpentinization. Methane is created by life as a waste product. Some animal life, such as cows and sheep, produce methane through microbes in their stomaches. These ancient archaean microbes, called methanogens, take in H₂ and CO₂ and expel CH₄ as waste. Like O₂, CH₄ will generally not stay in an atmosphere unless it is continuously replenished causing chemical disequilibrium. This is why the topic of methane in Mars’ atmosphere often pops up in the news, as it could be interpreted as a sign of life (but, if the detection is real, is very likely due to permafrost evaporating at the Martian poles).

Methane on its own would not be a strong biosignature but if it was detected with O₂ then a very strong case could be made for life, as this combination would be hard to create without photosynthesis.

Sulfur gases

Gaseous molecules containing sulfur are produced by both abiotic and biotic sources on the Earth. Simple sulfur gases such as SO₂ and H₂S are produced by metabolism but also are produced naturally though volcanic outgassing. A less ambiguous biotic signature is from more complex sulfur cases such as dimethyl sulfide (DMS) and dimethyl disulfide (DMDS). DMS on Earth is created mainly by marine phytoplankton and is also found in microbial mats.

The atmosphere of the exoplanet K2-18 b — a Super-Earth orbiting in the habitable zone around its host star — was recently studied by JSWT. When K2-18 b passed in front of its host star, K2-18, the infrared spectrograph on JWST collected the light from the planet and the star, and was able to separate out the atmosphere of just K2-18b, shown in Figure 11. DMS was detected but further studies will be needed to corroborate this detection.

Nitrous Oxide

Nitrous oxide (N₂O) is detected in excess (chemical disequilibrium) in the Earth’s atmosphere. It can be created from both abiotic and biotic sources. One biotic source is actually more of a technosignature — the amount of N₂O in our atmosphere has risen exponentially since the Industrial Revolution in 1750. It is a byproduct of agriculture and fossil fuel burning. It is also produced by bacteria that use nitrates in the soil and water to create N₂O. Nitrous oxide can also be formed naturally by lightning, but the bulk of N₂O in Earth’s atmosphere is due to life.

If the detection of N₂O is to be considered a biosignature, it must be considered along with other factors about the exoplanet and other gases that are found in its atmosphere.

Vegetation Red Edge

In addition to biosignatures gases in the atmosphere, a planet’s surface can also reveal the presence of life. On Earth, the red edge is a property of chlorophyll in photosynthetic plants, which becomes abruptly reflective at infrared wavelengths as shown in Figure 4. At wavelengths above about 0.7 μm (700 nm, which is right at the edge between red visible light and infrared light), leaves will reflect sunlight and show a sharp increase in the amount of light reflected at this point.

The specific wavelength for the red edge on Earth is a consequence of the properties of the Sun and the intensity of its light at different wavelengths (the most intense light from the Sun being yellow-green). For an exoplanet orbiting an M star, the most intense radiation is in the infrared part of the electromagnetic spectrum, so the red edge could actually be some different color or perhaps a black edge, depending how vegetation responds to the different intensity of light. H.G. Wells considered this and wrote in The War of the Worlds in 1895: “The vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint.”

This reflectance property of chlorophyll also keeps plants from overheating, but is also integral to plants growing efficiently through a process called shade avoidance. Plants are capable of detecting a lack of infrared light and this triggers a response in plants that produces longer stems and larger leaves, which allows a plant to capture more sunlight.

Photosynthetic organisms were a critical step in the evolution of aerobic respiration in eukaryotes. Therefore, a similar detection in the atmospheres of other planets would be an intriguing clue for bio-activity. The red edge is a strong signal in the atmospheric spectra from Earth and an easy feature to search for in the atmospheres of other planets.

The image to the left shows an example of an exoplanet, Kepler-186f, that is Earth-sized and falls into the habitable zone around its host star. Kepler-186 is a cool, red dwarf star so any plants that grow there could adapt to their star’s light in such a way that leaves have different colors that differ from those on Earth.

Review Questions

Summary Questions

1. What is a biosignature? Give some examples of Earth’s biosignatures.
2. How do SETI scientists search for biosignatures in planetary atmospheres?

Exercises

1. Assessing the habitability of an exoplanet. Choose an exoplanet that you are interested in learning more about, especially the possibility of it hosting life. Begin by hypothesizing whether or not you think the exoplanet is habitable. Go through the four factors (density, surface gravity, habitable zone estimation, and tidal locking) in the Assessing Exoplanetary Habitability section above and discuss whether the evidence you found supports the hypothesis.