Introduction
For centuries, humankind has been looking at the stars and wondering whether a world like ours could exist in the vastness of space. Our search has been stirred in recent years with the discovery of the first extrasolar planets, which proved that the galaxy is populated by billions of other worlds. We now have substantial evidence for the existence of three types of exoplanets (gas giants, hot super-Earths and ice giants), but life as we know it on any of these worlds would be impossible. The challenge that faces us now is to find a habitable terrestrial planet, which could potentially either harbour or sustain life.
This dissertation aims to explore the possibility of finding habitable terrestrial planets outside our solar system, concentrating specifically on recent and ongoing planet-hunting missions. In order to understand the missions’ scope, parameters and results, we must firstly introduce the idea of “habitable zone”, and the currently recognized parameters for habitability. We will then proceed to look at NASA’s Kepler mission, exploring its goals, methods and findings. Chapter 3 will briefly present a few of the confirmed Kepler planets that are believe to be among the best candidates for habitability. In Chapter 4, we will move on to considering the TRAPPIST and Spitzer telescopes, before discussing the recent discovery of seven Earth-sized planets orbiting the M-dwarf TRAPPIST-1, three of which are firmly set in its habitable zone. Finally, we will investigate other planned future missions, and the possible developments for this field.
1. Where to look, and what to look for: Goldilocks and the habitable zone
As mentioned in the introduction, our goal is to identify a terrestrial planet, roughly Earth-sized, in the habitable zone of its parent star, where the local temperatures would make the presence of water a possibility. Why are all of these prerequisites necessary to classify a planet as habitable? The first chapter attempts to answer this question, and give an overview of what characteristics we are looking for when analysing a planet’s atmosphere.
• What makes a planet habitable? Water, size and temperature
The first question that arises when approaching the quest for habitable exoplanets inevitably concerns the definition of “habitability”. What are the fundamental characteristics for the sustenance of life? In order to set the initial parameters for habitability, we must out of necessity look at the only example we have of a planet harbouring life: Earth.
Two are the main properties that set Earth aside from other planets in our Solar System, making it possible for life to exist: liquid water and the planetary environment that maintains it. All forms of life we know, with no exception, use carbon-based molecules with liquid water as a solvent. It follows that a practical approach to the question of habitability has seen the concept of habitable being defined no more as where life can exist, but as “where liquid water can exist”.
The presence of liquid water on the surface of a planet is predominantly determined by the stellar type (the size and effective temperature of the star the planet orbits), and the position of the planet’s orbit. The range of distance from a parent star that makes it possible for liquid water to exist is known as the stellar habitable zone. According to the Goldilocks’ Principle, a planet cannot be orbiting too close to the star, as it would turn into a boiling inferno, or too far from the star, as nothing would be left but a frozen wasteland. Its position must be “just right”, i.e. within the habitable zone (HZ) defined by Michael Hart as a region of space around a star where water in liquid form can be found on the surface of a planet.
The closely linked factors that control the presence of liquid ground water are the planet’s surface temperature and its atmospheric pressure. Earth’s thick atmosphere contains carbon dioxide, methane and water vapour that act as “greenhouse” gases, effectively insulating the surface in a way very similar to the panes of glass containing heat within a greenhouse. These gases allow visible sunlight through to heat the surface of our planet so that it begins to give off thermal radiation, while at the same time absorbing these shorter wavelengths of light, trapping the heat within the atmosphere. The greenhouse effect is an essential part of a life-sustaining planetary system: without it, Earth’s temperature would lower and our planet would freeze.
Once a life-sustaining atmosphere is formed, a planet must be able to hold on to it, and this ability is dependent on the planet’s size. Planets much smaller than Earth would not have enough surface gravity to hold on to a thick atmosphere; a thinner atmosphere would provide lower greenhouse heating, and the lower pressure would limit the temperature range of liquid water. On the other hand, a larger planet much larger than Earth (or super-Earths, with more than 10 Earth masses) would have much hotter interiors that would lead to high volcanic activity and a higher concentration of greenhouses gases, causing higher surface temperatures.
When all these factors are taken into consideration, we can conclude that the ideal habitable planet would have a mass of between 1/2 to 10 M⊕, a rocky composition, and lie in the habitable zone of its parent star.
• The HZ boundaries: CO2 condensation limit, greenhouse effect, stellar activity
We have seen that the HZ of a star is defined as the region around the star inside which a terrestrial planet can hold permanent liquid water on its surface, and how this depends mainly, but not exclusively, on the temperature and atmospheric pressure of the planet in consideration. An analyses of the Earth model in more detail highlights other factors without which life would be impossible.
Liquid water on Earth is made possible by a long term process of stabilisation of the surface temperature, closely linked to the CO2 levels in its atmosphere and the carbonate-silicate cycle. CO2 plays an essential role in maintaining the habitability of the planet, which led James F. Kasting and his colleagues to define the outer edge of a star’s HZ as the distance from the host star at which carbon dioxide starts condensing. CO2 condensation would not only disrupt the carbonate-silicate cycle, but would also form CO2 clouds which would increase a planet’s albedo (the amount of light which is reflected without being absorbed) by reflecting incident solar radiation and therefore cooling down the planet’s surface beyond reparation. Kasting defines this as the “1st condensation” limit, while the second limit is the maximum distance from the parent star at which a cloud-free CO2 atmosphere can maintain a temperature of 273 K: the “maximum greenhouse ” limit. On the other side of the HZ, the inner edge is defined as the distance from the parent star at which photodissociation breaks down water molecules in the stratosphere via UV radiation from the star into H2 and O, and hydrogen is allowed to escape from the atmosphere: the “runaway greenhouse” limit is indicated as the minimum distance from the parent star at which the atmosphere can maintain a process that allows the whole planet’s water resources to evaporate, therefore making it inhabitable.
The activity and evolution of the parent star are also fundamental variables to be taken into consideration. Main-sequence stars slowly increase in luminosity over their lifetime, therefore pushing the habitable zone away from the star, as it is itself a consequence of the star’s luminosity. Hence it is necessary to define a CHZ, or Continuous habitable zone, in which a planet may remain habitable for a considerable amount of time.
• On atmosphere characterization
Determining whether a planet lies in its host star’s habitable zone is only one of the steps to identifying it as a habitable planet. What we really need, to make a certain decision on the planet’s habitability, is an analysis of its atmosphere; to understand its composition and temperature. The goal is to individuate planets with an atmosphere similar to Earth’s, with water, vapour, oxygen and CO2: all the elements that we have identified in this chapter as necessary for the development of life as we know it. Astronomers can normally answer this question by using spectroscopic analyses, but this requires a direct view of the object, hard to obtain when studying exoplanets.
In recent years, however, astronomers have managed to extract the spectra of light passing through the atmospheres of exoplanets as they transit their parent stars. This method is called transit spectroscopy, and was first employed by David Charbonneau and his colleagues in 2002 to detect a sodium signal from a Jupiter-sized exoplanet transiting HD 209458, a star about 47 parsecs from Earth. It was both the first detection and the first spectroscopic measurement of an exoplanet atmosphere. Shortly afterwards, space-based transit observations were detecting gases such as carbon monoxide and water vapour.
An alternative method to analyse atmospheres consist in direct imagining: blocking out the starlight to see exoplanets in distant orbits and record their spectra directly. The first efforts to do this were unsuccessful, as even the dimmest parent star is much brighter than an exoplanet, but soon scientist discovered that it was possible if looking at brighter light sources such as young planets still glowing from the heat of formation, in orbits far from their stars. The first directly imaged exoplanets were simultaneously announced by two teams of observers in 2008. The spectra of the objects were obtained with the aid of adaptive optics, a technology that facilitates the detection of exoplanets around a star by correcting the twinkling of the star’s light caused by the turbulence in Earth’s atmosphere. Astronomers also used discs inserted into the telescope’s optical pathway to block the light from the star, and signal processors to digitally sharpen the images.
2. The Kepler mission
The end of the twentieth century marked one of the most revolutionary milestones in the history of exoplanetary exploration. In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two rocky planets orbiting a pulsar, PSR 1257 + 12, following the study of anomalies in its pulsation period detected using the Arecibo radio telescope. Only three years later, in October 1995, Didier Queloz and Michael Mayor announced the explosive discovery of the first exoplanet orbiting a main sequence star. 51 Pegasi b, as it was catalogued, was half the mass of Jupiter, and had a very short orbital period of 4 days. This was the opening of the flood gates. After it was confirmed that finding planets was a real possibility, all telescopes were pointed at the sky in search of other worlds, and new planets were discovered first in the dozens, then in the hundreds. The Kepler Space Telescope, the tenth in the series of NASA Discovery Program missions, was launched in 2009 by NASA to monitor 170,000 stars in our Milky Way Galaxy over a period of four years and determine the frequency of Earth-size planets in and near the habitable zone of Sun-like stars. In this chapter, we will report the mission’s specific goals, and look at which scientific method was used to achieve them.
• The goals: detecting habitable planets
As we have mentioned, the Kepler mission aims to explore the structure and diversity of planetary systems. More specifically, the goals of the mission as specified in the Concept Study Report are to: determine the frequency planets with a radius of 0.8 R⊕ (Earth-radii) and larger in or near the HZ of a wide variety of stars; determine the distributions of sizes and orbital semi-major axes of these planets; estimate the frequency and orbital distributions of planets orbiting multiple-star systems; determine the distributions of semi-major axis, albedo, size, mass and density of giant planets with short periods; identify additional members of each planetary system and determine the properties of those stars that host planetary systems.
• The science: transit detection method and instrument design
The Kepler telescope finds planets by using the transit detection method. This involves observing repeated transit of planets in front of their stars, which causes a slight dip in the star’s apparent magnitude, on the order of 0.01% for an Earth-size planet.
Figure 1: Light Curve of a Planet Transiting Its Star
Image Credit: NASA Ames
Once the planet has been detected, the depth of this reduction in brightness can be used to deduce the size of the planet, and its orbital size can be calculated from its period and the mass of the star, using Kepler’s Third Law of planetary motion. Using the orbital size and the temperature of the star, the planet’s characteristic temperature can be calculated. Once all of this data has been collected, we can start making deductions on the planet’s habitability.
The Kepler Mission is absolutely reliant on exact differential photometry, to detect the tiniest dips in brightness of the observed stars. The Kepler instrument is therefore a specifically designed Schmidt optical system with 0.95-meter aperture, a 105 square degrees field-of-view, and an array of 42 CCD detectors at the prime focus. Each 50×25 mm CCD has 2200×1024 pixels, and is read out every three seconds to prevent saturation.
Data from the individual pixels that make up each observed star are recorded continuously and simultaneously, stored on the spacecraft and transmitted to the ground once per month. The photometer must be space-based to obtain the precision needed to reliably see an Earth-like transit and to avoid interruptions associated with ground-based observing.
Figure 2: Photometer Cross-section – Labelled Figure 3: Kepler Flight Segment – Labelled
Image credit: NASA/Kepler mission Image credit: NASA/Kepler/Ball Aerospace
3. Kepler’s planets: the best candidates for habitability
As of March 2017, Kepler has discovered 2,331 confirmed exoplanets, but only a precious few are roughly Earth-sized and reside in the habitable zone of their parent star. This chapter briefly presents some of the most exciting candidates for habitability.
Figure 4: Kepler’s Small Habitable Zone Planets
• Kepler 62e and 62f
In April 2013, NASA announced the discovery of the Kepler-62 system, a five-planet system about 368 parsecs from Earth in the constellation Lyra. The five planets of Kepler-62 orbit a seven billion years-old K2 dwarf, measuring just two thirds the size of the Sun and only one fifth as bright, which places the HZ limit relatively close to the star. Kepler-62 is home to two habitable zone worlds, Kepler-62f and Kepler-62e. With a radius of 1.4 R⊕, Kepler-62f is classified as a super-Earth, orbiting its parent star every 267 days, and with temperatures that might be around 284–290 K. Given its size and age, it is plausible that the planet is of rocky composition and presents a substantial amount of water, but the distance from its star means that Kepler 62f would need a significant amount of CO2 in its atmosphere to avoid being covered in frozen ice. The other habitable zone planet, Kepler-62e, orbits much closer to the star, with a period of 122 days, and is roughly 60% larger than Earth. The two habitable zone worlds orbiting Kepler-62 have three interior companions, two larger than the size of Earth and one about the size of Mars. Kepler-62b, Kepler-62c and Kepler-62d, orbit every 5, 12, and 18 days respectively, making them very hot and inhospitable for life as we know it.
• Kepler 186f
Discovered in 2014, Kepler-186f is the first Earth-size planet discovered in the potentially habitable zone of a star. Orbiting a main-sequence M1-type dwarf star with a temperature of 3788 ± 54 K and an iron abundance half that of the Sun, Kepler-186f has an orbital period of 129.9 days and a planet-to-star radius ratio of 0.021. Kepler-186f resides in the constellation Cygnus, 171 parsecs from Earth, and the Kepler-186 system is also home to four inner planets, seen lined up in orbit around a host star that is half the size and mass of the Sun. As outlined in Elisa Quintana’s study of the planet, “the intensity and spectrum of the star’s radiation places Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and H2O at its surface, then some of this H2O is likely to be in liquid form”. Kepler-186f is likely to be of rocky composition and generally assumed to be a good candidate for potential habitability.
• Kepler 442b
Kepler 442b is a super-Earth orbiting a K-dwarf with 11% of the Sun’s luminosity. With a radius of 1.34 R⊕, Torres et al. estimate Kepler 442-b to have a 60.7% probability of being a rocky planet and, even assuming a conservative definition for the outer limit of the HZ, this world seems to have a very high probability of orbiting comfortably inside this zone.
• Kepler 452b
Kepler-452b, sometimes nicknamed Earth 2.0 or Earth’s Cousin, is an exoplanet orbiting the Sun-like star Kepler-452 about 430 parsecs from Earth, in the constellation Cygnus. Its discovery, announced by NASA in July 2015, caused particular excitement as it was the first super-Earth planet with a potentially rocky composition discovered orbiting within the habitable zone of a sun-like star.
4. The TRAPPIST -1 system
• From TRAPPIST to SST: a gradual discovery of the planetary system
TRAPPIST (Transiting Planets and Planetesimals Small Telescope–South) is a reflecting telescope of 60cm in aperture diameter, situated in ESO’s La Silla Observatory in Chile, which had first light in 2010. The telescope is controlled from Liège, in Belgium, as joint venture between the University of Liège and Geneva Observatory, in Switzerland. Amongst other tasks, TRAPPIST specializes in the study of planetary systems by detecting and characterizing exoplanets utilizing transit photometry.
In May 2016, a team headed by Michaël Gillon of the University of Liège announced the discovery of three Earth-sized planets orbiting the ultracool M-dwarf TRAPPIST-1, 12 parsecs away. As reported in the Nature article, that announced their discovery, the international team of astronomers had used TRAPPIST to monitor the star in the very-near infrared for 245 hours over 62 night at the end of 2015, and detected 11 clear transit-like signatures with an amplitude close to 1%. With the aid of a plethora of telescopes around the world (HCT, VLT, UKIRT), they managed to attribute the signatures to two planets: TRAPPIST-1b and TRAPPIST-1c, orbiting the star very close with orbits of 1.51 days and 2.42 respectively, and a third planet, TRAPPIST-1d, for which 11 orbital periods – from 4.5 days to 2.8 – were found possible. Given their short orbits, the planets were declared likely to be tidally locked, rotating around the star but not on their own axis, which leads to a world where one half is constantly in the light, while the opposite half is in a state of perpetual darkness. Between the two opposite sides lies the terminator line: a twilight zone which is most likely to present temperatures suitable for liquid water. With a habitable zone estimated at between 0.024 and 0.049 AU from the star, TRAPPIST-1b and 1c were found to be too close to the star to be habitable, even if their temperature is low enough that they could have habitable regions at the western terminators of their day sides, while TRAPPIST-1d lies just within or slightly beyond the HZ.
The discovery of planets orbiting an ultracool dwarf was a paradigm shift in the field of planetary hunting, as until that moment the existence of “red worlds” orbiting faint red stars was purely theoretical. TRAPPIST proved it was possible, and that such a planetary system was available to study only 12 parsecs away. The enthusiasm following this discovery caused a number of telescopes to be pointed at TRAPPIST-1, including NASA’S Spitzer Space Telescope.
Spitzer is an infrared telescope, originally launched in 2003 as the fourth and last of NASA’s Great Observatory Missions, with a mission scope of 2.5 years, possibly extendable to 5 years, depending on how long the on-board supplies of liquid helium, necessary to keep all instruments within the Cryogenic Telescope Assembly cool enough (i.e. just a few degrees above absolute zero), would last. The coolant finally depleted 2009, which is when the Spitzer “warm mission” was initiated: if many of the on-board instruments were no longer fit to be used, Spitzer was still able to perform different functions, including the use of transit photometry and gravitational microlensing to study exoplanets.
Figure 5: The Trappist system: initial finding vs. current status
Between September and October 2016, Spitzer was used to observe TRAPPIST-1 in a 20-day photometric monitoring. As reported in the team’s research paper published on Nature in February 2017, the resulting light curve showed 34 clear transits which, when combined to the ground-based observation, were identified as transit signals from four new planets named , TRAPPIST-1d, TRAPPIST-1e, TRAPPIST-1f and TRAPPIST-1g, with period of 4.04 days, 6.06 days, 8.1 days and 12.3 days respectively. The Spitzer results also showed a separate transit-shaped signal with a depth of around 0.35% and a duration of 75 minutes, attributed to a seventh outer planet of unidentified period: TRAPPIST-1h. In less than a year, we went from a three-planet system orbiting an ultracool dwarf to a planetary system of seven planets, three of which are estimated to lie in the habitable zone of TRAPPIST-1.
Never before had astronomers been able to identify such a large number of potentially rocky planets orbiting one single star outside our solar system, with equilibrium temperatures low enough to make possible the presence of liquid water on their surface. This is an exciting discovery in the search for life on other worlds: it is now possible that future study of this as yet unique planetary system could reveal conditions suitable for life.
• The planets in the HZ: TRAPPIST-1e, TRAPPIST-1f and TRAPPIST-1g
The three planets that caught the world’s imagination upon NASA’s announcement of the recent discovery within the TRAPPIST-1 system are the worlds that lie in the star’s habitable zone where, with the correct conditions and atmospheric properties, liquid water may be present on the surface. There is also a high probability that the planets are tidally locked to their parent star, and extreme weather patterns and temperature changes might be a consequence of this.
Figure 6: Artist’s illustration of the TRAPPIST-1 System
TRAPPIST-1e, TRAPPIST-1f and TRAPPIST-1g are all Earth-sized exoplanets. TRAPPIST-1e has a radius of around 0.92 R⊕, which makes it highly likely to be rocky, but also tidally locked, with a terminator line where the temperature could be suitable for liquid water. Furthermore, if the planet were to have a thick enough atmosphere to transfer heat to the “dark side”, a much larger portion might be habitable. TRAPPIST-1e has a mass of 0.62M⊕, a temperature equilibrium of 251K, and an orbital period of 6 days.
TRAPPIST-1f has an equilibrium temperature of 219 K, a radius of 1.04 R⊕ and a mass of around 0.68 M⊕. This values allow to estimate the gravity to be 6.17 m/s2, a 63% of Earth’s. TRAPPIST-1g has a mass of 1.34 M⊕ and a radius of 1.13 R⊕., which allows us to estimate a gravity of 10.3 m/s^2 and a density of 5.13 g/cm^3, values similar to those of Earth. TRAPPIST-1g has an orbital period of about 15.353 days and an orbital radius of about 0.045 times that of Earth.
5. Future developments and planned missions
According to astronomers, the future of exoplanet exploration relies upon direct observation. This will be possible once missions like the James Web Space Telescope, under construction, are launched, increasing our ability to direct-image distant stars and planets. The future NASA mission Wide Field Infrared Survey Telescope (WFIRST) mission, for example, is an infrared space observatory that will use a coronagraph to block light from the star and reveal the planets that are usually too faint to be detected in full starlight.
Meanwhile, a collaboration between the University of Liège in Belgium, the Cavendish Laboratory in Cambridge and the King Abdulaziz University in Saudi Arabia is developing a project named SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) that will consist of four Ritchey-Chretien design telescopes of 1m primary aperture, each equipped with a NTM-1000 robotic mount and cameras sensitive in the near-infrared, the wavelength range in which ultra-cool stars and brown dwarfs release most of their light. The scope of the project will be to search for Earth-sized planets around 1000 ultra-cool stars and brown dwarfs over five years.
Conclusion
This dissertation has explored the possibility of finding habitable worlds outside our own. We have analysed the meaning of “habitable” in connection to the presence of water, temperature and mass, and the characteristics necessary for a planet to be able to hold on to a life-sustaining atmosphere. We have then looked at the recent and ongoing planet-finding missions, and their exciting results.
In conclusion, we can now ascertain that it is indeed possible to find worlds outside our solar system that are Earth-size, likely to be rocky, and lying in the habitable zone of their parent star. In fact, as the recent discovery of the TRAPPIST-1 system has confirmed, it is possible to find a whole system of such planets, three of which lie in the HZ of the parent star.
Our scientific effort must now concentrate on finding a way to observe these planets directly, analyse their atmosphere, and confidently determine their habitability. In the last chapter of this thesis, we have discussed the planned and future mission which will employ new and developing technology to make this possible. Once all the data is in hand, and we can determine which of the candidate planets are suitable for human life, all is left is to figure out a way to cross the vastness of space, and get there.
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