The ancient Greeks were the first to think the possibility of civilization in the Universe and until today the search for extratterestial life seeks to find the answer to the same question: Are we alone in the Universe? Astonomers are using various techniques and instruments from the Earth and space to retrieve information from the atmospheres of Earth-like planets the so-called super-Earths to find signs of life. This dissertation describes super-Earths, how Astronomers detect biomarkers in their atmospheres and whether they can support life.
Since 1995, scientists, in their search for extraterrestrial life, have discovered 3781 exoplanet1, 2781 planetary systems of which 1233 planets are characterised as ‘‘super-Earths’’ that could support life. Super-earths are bodies orbiting stars in the Universe and their mass is 1-3 times that of Earth. From observations, Astronomers found that they are either rocky or water worlds and that’s why they have been described like Earth. The first super-Earth was discovered by Rivera et al. (2005)2, orbiting a star named GJ 876. To detect them, they use various techniques such as the transit method, radial velocity method, microlensing and direct imaging which also helps them understand their size, mass, atmospheres and composition.
In order to understand if super-Earths can support life we first need to answer the following questions:
– What are the ingredients of life?
– What are super-Earths?
– How do Astronomers detect them and observe potential biomarkers in their atmospheres?
This dissertation will be an attempt to research and discuss the aforementioned questions in more detail.
“In some worlds there is no sun and moon, in others, they are larger than in our world, and in others more numerous. The intervals between the worlds are unequal; in some parts there are more worlds, in others fewer; some are increasing, some at their height, some decreasing; in some parts they are arising, in others failing. They are destroyed by collision, one with another. There are some worlds devoid of living creatures or plants or any moisture.” Democritus (~460-370 B.C.)
The definition for life has been in discussion throughout history and different ones have been proposed in the field of biology, biochemistry, genetics, astrophysics and others. Scientists today are mainly focusing on the nature and attributes of life, as opposed to find a definition about the word ‘life’ as we understand it. So what is life? One interesting definition from NASA is that ‘’Life is a self-sustaining chemical system capable of Darwinian evolution’’34.
The Universe is abundant to organic chemicals such as carbon, hydrogen, nitrogen, oxygen, sulfur and phosphorus. Astronomers use spectrometers to detect the emission lines and the abundance of the aforementioned elements in planets, to help them understand their composition and find signs of life.
Life as we know it is based on carbon atoms and when combined with other atoms can create complex molecules. An alternative to how life might have arisen is the silicon-based atoms which are similar to carbon atoms. For example, both can form molecules with four hydrogens and both can form polymers, but silicon atoms can’t create complex systems like carbons do. Comets and asteroids hit planets and can carry carbon-based molecules in a collision with a planet can bring these with them5.
So the building block of life is atoms. To prove that chemicals can form the building blocks, scientist Miller- Urrey, carried out an experiment. In a shielded container, they combined chemicals such as hydrogen (H2), ammonia (NH3), methane (CH4), water (H2O) and then expose them to simulated lightning for a week. The result was the formation of rich amino acids, thus, scientists made the assumption that because chemicals can form easily so life can arise as a result5.
What about the cell structure of organisms? Terrestrial organisms differ from each other and this is because of the structure of their cells. Life on earth is based on either the Procaryotic (eg. bacteria) or Eucariotic cell structure (plants, fungi). Multi-cellular life started from 15 billion years ago, eukaryotes are multi-cellular, thus it is assumed that throughout Earth’s history they have undergone environmental and natural selection changes and became more complex systems6.
Another common attribute that terrestrial organisms share is the double-helix deoxyribonucleic (DNA) storage and ribonucleic (RNA) genetic molecules. Their function is to create copies of the genetic code. What happens is that the double helix of the DNA is unzipped by a polymerase RNA enzyme and separates it in halves that allow them to be rebuilt. This process creates two DNA copies. The transcribed RNA can use the information stored in the DNA to create new proteins (polymer molecules)6.
The formation and evolution of the multi-cellular systems coincided with the oxygenation of the Earth’s atmosphere. What happened is that the atmosphere changed from being carbon dioxide to oxygen rich and the terrestrial organisms utilized this chemical process. This was the result of photosynthetic life with the main molecules cyanobacteria – also called blue-green bacteria- which they are using stellar photosynthesis to produce oxygen. Natural selection – the process that organisms tend to evolve and adapt to their environment- drove to oxygenic photosynthesis6.
Cyanobacteria have been on Earth since the crust was formed and water existed. As opposed to other organisms that we know that have gone on extinction due to catastrophic events -such as asteroids crashing on Earth or atmospheric changes- cyanobacteria have survived. Based on this, it is possible to assume that the life we might find in exoplanets, will be in the form of bacteria6.
Water is essential to life on Earth and it must be liquid5. The human body is made up of 60% water and there are other terrestrial organisms that are made up of 90% water7. For water to remain liquid the temperature and pressure have to be at a certain level. For example at a pressure of 661 Pa (Pascal) and a temperature of 273K (Kelvin) the water can be liquid, vapour and ice, but below this pressure, there is no liquid water and at higher temperatures the pressure controls the behavior of the water and change to solid 6.
Habitable Zone (HZ) is the defined orbital region around a star where a planetary body has liquid water and therefore capable to sustain life. As well, for life to exist on a planet it is essential to have an atmosphere because the pressure and temperature regulate the conditions (not too hot and not too cold) for liquid water to exist 6.
The question is can life exist where there is no water? The answer could be yes. A classic definition8 for the HZ is that for a planet to sustain life it must have an Earth-like atmosphere and that the presence of water ‘’is not impossible’’, which implies that a planet in a habitable zone can either have water or not 9.
Observations from one of Jupiter’s moons Europa show us that has an ocean of water under the icy surface5. Other observations have shown that the liquid lakes of Titan are methane and ethane and for Astrobiologists, that is an indication that extreme environments for organisms might exist in other planets 6.
However, the concept and definition of the HZ are continuously evolving. The planet’s surface and the temperature are dependent upon the albedo, atmosphere, the greenhouse effect and other energy sources such as radioactive decay and tidal heating6. These can support the planet to maintain a subsurface where there could be liquid water, thus can sustain life. For example, Europa has liquid water beneath its surface, thus it might be habitable. Other examples are extrasolar planets such as Proxima Centauri b and three planets in the Trappist – 1 system that are Earth-sized and have been found to orbit stars in the HZ.
In addition, a planet’s HZ is dependent upon the luminosity and mass of the star. A star that is too young is hotter and if the planet is in the HZ might be too hot and or too cold (see image 1) for water to exist. A main sequence star’s luminosity increases with time and the atmospheric conditions change, so the location of an Earth-like planet would be in the “continuously habitable zone”, where a planet is habitable during the Main sequence lifetime. This zone is from 0.9 to 1.2 Astronomical units (AU)10.
Moreover, a high mass-star lifetime is millions of years but we know that for life on Earth to evolve it took billions of years, therefore it is unlikely that an extrasolar planet orbiting a high-mass star would exist long enough for life to emerge.
Image 1 Credit: NASA
Fainter stars such a dwarf-stars last trillion of years but they emit infrared radiation which makes it difficult for life to exist. The planet’s HZ would be closer to the star and tidal forces would cause the planet to face one side, thus it would be either night or day and those planets’ atmosphere will either freeze or distribute the heat10.
Super-Earths are extrasolar planets that are larger than Earth but smaller than planet Uranus that is 15 times larger than Earth. A super-Earth has a mass between 1-10 times the mass of the Earth. Kepler’s mission defines super-Earths as Earth-like planets with a radius two times smaller that of Earth’s radius. The first super-Earth named Gisele 876 d was discovered in 2005 orbiting a star GJ87611.
According to the super-Earths’ density -that is calculated from the mass and radius measurements- their composition can be that of rocks, water and or hydrogen and helium sometimes described as mini-Neptunes. Hubble Space Telescope (HST) observations have shown evidence that water planets can have clouds in the atmosphere such as the GJ121412.
In the search for life, astronomers are mainly focusing on super-Earths that are Earth-like rocky planets or ocean worlds – these are planets that may harbour liquid oceans or lakes- in the habitable zone of the stars where extraterrestrial life in the form of organisms can exist.
The first super-Earth was discovered by Rivera et al (2005)2 using the radial velocity method and the first super-Earth detected using transit photometry orbiting an M star in the habitable zone was GJ 1214 b 13. And three of the first terrestrial super-Earths discovered in the habitable zone was GL 581 d14 , GL 581 g15 and GJ 667C c16.
In the recent years, Kepler’s Space Telescope has discovered the following super-Earths that are described as ocean worlds 17:
Kepler-22 b (see image 2) is the first planet detected in a star’s habitable zone where water could exist on its surface and it is two times that the Earth’s size.
Image 2 Credit: NASA/Ames/JPL-Caltech
Kepler 62 has 5 planets orbiting a star and two of the planets Kepler 62f and Kepler 62e are located in the habitable zone. Kepler 62f orbits the star every 267 days and it is 40% larger than Earth.
Astronomers use various methods for the detection of exoplanets and super-Earths such as pulsar timing, radial velocity, astrometry, transit photometry, radial velocity, microlensing and direct imaging, to name few18. These techniques can provide information about the characteristics of a planet that in turn contribute to the understanding of the planet and the planetary system as well.
For example, in the transit method, they observe from Earth-ground based telescopes exoplanets that pass in front of a star. During this transit, the light from the star decreases and some of the light passes through the planet’s atmosphere. From this observation, the planets transmission spectrum can be measured, as well its size and period if the radius of the star is known19.
The radial velocity method has the same effect as the Doppler shift where the frequency or wavelength of the light is changing. So, the light of the star of a planetary system does not change but its frequency changes. The star’s radial velocity can be measured when the star is moving towards us and away from us. From this motion, Astronomers can find if there is a planet orbiting the star, its mass and orbital period 18 and thus helps them to detect Jupiter-like planets and super-Earths.
The transit and radial velocity method can be used in conjunction to obtain information about the planets mass, size and the density of the planet, therefore their composition, whether they are rocky planets or ocean worlds18.
Earth’s lower atmosphere is composed of molecular Nitrogen N2 (78%) and Oxygen O2 (21%) which the most abundant elements. Nitrogen does not escape Earth’s atmosphere thus it is one of the main components. Oxygen is produced by photosynthesis by plants and organisms. The greenhouse gases are chemical compounds of Carbon dioxide CO2, Methane CH4 and Nitrous oxide N2O and are the ones that heat the Earth’s atmosphere. An important molecule on Earth’s atmosphere, Ozone O3, is generated via “photodissociation” -– the breakup of molecules- of O2 and it is found in Earth’s stratosphere19 18. When the aforementioned molecular elements found in a planet’s atmosphere indicate that life can exist and Astronomers have named them biomarkers.
In contrast, the atmospheres of gas giant planets such as Jupiter and Neptune are composed of hydrogen, helium and elements of Carbon, Oxygen, Nitrogen, Sulfur that form other molecules CH4, H2O, NH3, hydrogen sulfide (H2S) and phosphine (PH3) where life can’t exist.
To understand the evolution of the atmospheres and obtain data from the Earth and other solar system planets Venus, Mars, Jupiter, the Moon, Titan, Mercury, Enceladus, Astronomers use remote sensing techniques or by probes in situ with mass spectrometers and or in close orbit18 20. Thus, to understand the biomarkers of super-Earths is important for Astronomers to look into their atmospheres that are responsible for the planetary spectrum features. The question that arises is how do they retrieve data from exoplanets and super-Earth atmospheres?
The two main methods for radius measurements are absorption spectroscopy and emission spectroscopy:
Absorption spectroscopy is when the light of a star passes through a planet’s atmosphere during the first transit which is then captured and analyzed by a telescope and can reveal its constituents such as its atmospheric gases.
Emission spectroscopy is when the planet is behind the star and only the light from the star can be seen which can reveal the upper layer of a planet’s atmosphere and the thermal emission of the planet.
The difference of the two signals can show Astronomers the spectrum of a super-Earth and by analyzing it they can get data about the atmospheric composition21.
In 2016 seven exoplanets were detected 40 light years away from Earth, orbiting an ultracool dwarf star, the TRAPPIST 1 (see image 3), with Jupiter-like mass, and orbital periods 1.5 to 12.4 days and 20 days the outer planet. To obtain data, they used photometric measurements from the ground telescopes, the Transiting Planets and PlanetesImals Small Telescope (TRAPPIST), a robotic telescope at the European Space Agency’s (ESA) La Silla Observatory in Chile and the Spritzer Space telescope. They analyzed the data of the transit light curve to find the depth, duration and timing. From the radius measurements of the inner planets, they found their densities that indicated rocky compositions. Three of them were found to be in the habitable zone 22.
On May 2016 NASA’s Hubble telescope took the first spectroscopic measurement of the two TRAPPIST 1 planets b and c in a simultaneous transit that occurs every two years. They filtered the light coming from the two planets the moment both passed in front of the star and the signals received indicated that most probably don’t have hydrogen-rich atmospheres -which is found in gaseous planets such as Neptune- and potentially they could support life23.
Image 3 Credit: NASA
Another team of scientists, in their study, used the solar wind of the star to simulate the corona and the wind using the latest data and showed that the outer planets could retain their atmosphere and potentially can support life24.
In a new recent study25, Astronomers used from NASA’s Spitzer and Kepler Space telescopes the near-infrared (1.1–1.7 μm) G141 grism on the Wide Field Camera 3 (WFC3) instrument to obtain new data (Atmospheric reconnaissance) and measure the planets’ orbital period and densities. With the new measurements, they ran a simulation and found that all seven planets are mainly made of rock and only 5% percent of water. The existence of water in the planets is important for habitability. They showed that three of the planets, d, e and f have no sign of hydrogen-rich atmosphere which indicates that they have compact atmospheres similar to Earth26. A description of the three planets published on Jet Propulsion Laboratory’s website:
“TRAPPIST-1b, the innermost planet, is likely to have a rocky core, surrounded by an atmosphere much thicker than Earth’s. TRAPPIST-1c also likely has a rocky interior, but with a thinner atmosphere than planet b. TRAPPIST-1d is the lightest of the planets — about 30 percent the mass of Earth Scientists are uncertain whether it has a large atmosphere, an ocean or an ice layer — all three of these would give the planet an “envelope” of volatile substances, which would make sense for a planet of its density.”27
Five other exoplanets were discovered orbiting a star named 55 Cancri (see image 4), in the Cancer constellation approximately 40 light years from Earth. The planets were detected with the radial velocity method. One of the planets 55 Cancri e is a super-Earth orbiting its star very close with period 0.7365 days, mass 8 times that of Earth and a surface temperature that can reach 2000K. The planet’s transit was observed with the Spitzer and MOST Space Telescopes28.
Super-Earth 55 Cancri Credit: ESA/Hubble
A team of Astronomers at University College London (UCL), analysed the spectra data of the planet and for the first time gases were detected in the atmosphere of a super-Earth. For the spectroscopic observations, they used data from the Wide Field Camera 3 (WFC3) on Humble’s Space Telescope (HST), which has been used before to detect the atmospheres of two other super-Earths GJ1214b and HD97658b that suggested an atmosphere “heavier than hydrogen”. So they analysed “two HST/WFC3 scanning –mode spectroscopic observations” which were further analysed through a computer software using the so-called T-Rex retrieval code. According to their results, the planet has an atmosphere with a significant amount of hydrogen and helium from the protoplanetary disk during the planetary system’s formation and has no water vapor28 and therefore cannot support life.
Another team of Astronomers at Keel University in the United Kingdom have detected atmosphere on a super-Earth 39 years light away named GJ1132b orbiting very close to an M-dwarf star thus it’s very hot and cannot be habitable. The planet has 1.6 the mass of Earth which makes it very small compare to Cancri 55e and temperature that can reach 650K. To obtain the light curve data and determine the radius of the planet, they used the optical and near-infrared transits photometry method and observed the planet’s transits in front of the star using the GROND multiband imager telescope at ESO La Silla in Chile. Team members from the University of Cambridge and the Max Planck Institute for Astronomy simulated the atmospheres using the data acquired for the mass and radius which showed an Earth-like thick atmospheric composition which can be either rocky or an ocean planet. According to the paper results, they found that the planet’s measured mass and radius confirm their predictions of the existence of silicates or water and 100% iron composition29.
Until today scientists have used different methods to search for life in planetary systems including ours. Here are some of the known methods18:
- Searching for extraterrestrial intelligent life on Earth, for example, by studying the fossils of meteorites.
- Sending robotic probes in situ to retrieve samples and analyse them in onboard labs.
- Detecting biomarkers by analyzing the spectra of molecules in the atmospheres that are produced by living organisms.
- Looking for objects in our solar system that would indicate intelligent civilization are trying to communicate with us.
- Detecting radio signals sent by a technologically advanced civilization.
In 1976, NASA had sent to Mars the Viking Lander 1 and Viking Lander 2, in different locations, to test Martian samples of microorganisms from the surface and analyse them in an onboard lab to find life. The results were inconclusive and one of the reasons might have been that the microorganisms could not have survived in the surface locations the landers have landed and or that the Lander wasn’t advanced enough to retrieve samples deeper in the surface5.
Around the same period, one of NASA’ scientists James Lovelock formed the Gaia hypothesis and he suggested that living organisms interact with other elements and that basically, the biosphere integrates with the atmosphere which forms the complex system that maintains the conditions on Earth for life.30 31
It was since that time that Astronomers started looking into the atmospheres of our solar system planets and in exoplanets to find extraterrestrial life and understand its evolution. Today technologically the only way for the detection of gases in exoplanetary atmospheres is possible with the use of ground and space telescopes.
The detection techniques used by Astronomers to identify biomarkers in super-Earths, like the TRAPPIST-1 b, c, d and 55 Cacnri e planets, aren’t sophisticated enough and therefore can’t retrieve the necessary data to find biomarkers like oxygen, ozone, methane and carbon dioxide combined that are produced by living organisms and would indicate signs of life.
In the next few years, the ground-based ESO’s Extremely Large Telescope (ELT)32 and ESA/NASA’s James Webb Space Telescope (JWST)33 are going to be launched and will help Astronomers to look deeper into the exoplanets’ atmosphere to detect biomarkers and understand if there are signs of living organisms. Another important mission, scheduled to launch in March 2018 is the Transit Exoplanet Survey Satellite (TESS)34 that is going to look for the brightest stars to find exoplanets and to follow-up observations from JWST.
Life, as we know it on Earth, is versatile, and in order to understand how life can exist on other planets, Astronomers are first looking into how life has evolved on our solar system. Living organisms produce oxygen and methane which then can interact with other elements that produce gases in the atmospheres. These gases can be detected in the habitable zone of super-Earths and indicate signs of life. The detection methods of spectroscopy are currently the main tools to analyse the filtered light that’s coming from the exoplanets when transiting their star. Super-Earths have been in favour of finding biomarkers in the atmospheres because of their size as they are easier to detect with ground and space telescopes. Astronomers are eagerly waiting for the launch of the new technologically advanced ground and space telescopes, JWST, ELT and TESS that will help them in their ongoing research to confirm what is already predicted from their studies and find exoplanets and super-Earths that can support living organisms either as we know it or in extreme environments.
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