May 26, 2022
IV. results of exoplanet searches to date
The dedicated search for exoplanets in the Milky Way “neighborhood” of our solar system, especially since the 2009 launch of the Kepler Space Telescope, has paid enormous dividends. In late March 2022 NASA announced the milestone of 5,000 exoplanet discoveries now confirmed. As summarized in Fig. IV.1, those discoveries span a range of planetary types, including 4% that appear rocky and of similar size to Earth, and another 31% that are intermediate in size between Earth and Neptune, some of which may also be rocky. The ensemble provides an initial basis for learning about planetary formation processes, for narrowing down future searches for potentially habitable planets, and for statistical estimates of how many Earth-like planets may exist in the Milky Way galaxy alone.
The more detailed distribution of the discovered exoplanets with respect to orbital period, mass and radius is shown in Fig. IV.2. The color coding in the figure indicates which of the techniques described above in section III was used to make the discovery of each planet. It is clear that a majority have been discovered by the transit method, and particularly by the Kepler telescope. However, most of the planets seen in transit in front of their stars have not yet been analyzed by other methods, and do not yet have measured masses, so they appear only in the right-hand plot in Fig. IV.2. By the same token, most of the planets discovered by the radial velocity technique appear only in the left-hand plot because they have not been seen in transit, so they do not yet have measured radii.
Planets in our own solar system are indicated by red dots in both plots in Fig. IV.2. Their locations make clear one of the limitations in all exoplanet searches to date: because they have searched over a small number of years for periodic influences on the stars the planets orbit, their sensitivity has typically been limited to orbital periods shorter than those characteristic of similar size or similar mass planets in the solar system. As we will discuss later, that limitation is anticipated to be relaxed by microlensing surveys with the future Nancy Grace Roman (previously WFIRST) space telescope.
Although each exoplanet detection technique has its own technical sensitivity limitations, it is nonetheless clear from Fig. IV.2 that the Jupiter-like gas giants form a distinct class well separated from lighter or smaller exoplanets. These gas giants extend up to about ten times Jupiter’s mass; at much greater masses a gas giant would have a hot enough core to burn hydrogen and turn into a star. It is also clear from the figure that a large fraction of the exoplanets populate a class intermediate between Earth and Neptune – the so-called Super-Earths in Fig. IV.1 – not seen in our own solar system. But within that class there appear to be two distinct sub-populations whose separation is hidden in Fig. IV.2 within the dense cluster of Kepler discoveries. The gap within the “super-Earths” is seen in Fig. IV.3, which focuses on planets discovered by the Kepler telescope surrounding Sun-like stars with orbital periods less than 100 days. Although densities have not yet been measured for many of these planets, presumably the smaller-radius peak in Fig. IV.3 corresponds to rocky planets with thin atmospheres, while the larger peak includes “mini-Neptunes” with much thicker atmospheres.
At the time of the 2018 National Academies Exoplanet Science Strategy, there were a total of 418 transiting exoplanets whose masses, as well as radii, had been measured with better than 20% precision, allowing a determination of their mean densities. Their masses and radii are shown in Fig. IV.4 along with expected correlations for simple planetary compositions. Those that lie like Earth along the “pure rock” curve are candidates to be probed for habitability.
Indeed, a number of exoplanets of size comparable to or somewhat larger than Earth have been located by the Kepler telescope within the habitable zones of M, K, and G (Sun-like) class main sequence stars, as shown in Fig. IV.5. In this figure the habitable zone is indicated by the energy that should be received by each planet in its measured orbit from its local star. In our solar system both Earth and Mars lie within, but near opposite ends of, the habitable range. Although significantly more potentially habitable planets have been discovered in the six years since Fig. IV.5 was created, including around Sun-like stars, it remains true that the majority found so far orbit relatively dim M-class stars. Habitable zone planets around M-class stars have much smaller orbits and orbital periods than Earth, as indicated in Fig. IV.6, which compares orbits for two of the exoplanets in Fig. IV.5 to those of solar system planets. The predominance of M-class stars in the survey to date may reflect the distribution of stars within Kepler’s field of view and the orbit period sensitivity of analyses of Kepler data, more than a general Milky Way feature.
There are by now enough exoplanet discoveries and characterizations to support initial statistical extrapolations to estimate how many Earth-like planets within habitable zones may exist in the Milky Way. From the observations to date, it appears that there is approximately one planet per star (some stars hosting a multi-planet system), on average, in the Milky Way, or perhaps 200 to 500 billion planets overall. If, as in the sample summarized in Fig. IV.1, 4% of these are Earth-like planets in habitable zone orbits, that would suggest 8 to 20 billion potential other Earths. Some estimates go as high as 40 billion. Roughly a quarter of these may be further restricted to orbit Sun-like stars. Multiplied by the 100 billion or so galaxies visible in our universe, these estimates suggest hundreds of billions of billions of Earth-like planets. Not all of these may be capable of supporting life, but one would expect some to. So, where is everybody?
The most interesting system of exoplanets yet discovered surrounds an M-class star called TRAPPIST-1 located 41 light-years from Earth, a “stone’s throw” on galactic scales. The system is close enough that a ground-based telescope, the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, made the first exoplanet discoveries in the system. More were discovered by the Spitzer space telescope and data from Spitzer and other ground-based observatories allowed determination of the exoplanet masses, as well as sizes. More detailed analyses have since refined the determination of the exoplanet densities. The TRAPPIST-1 system contains seven planets tightly stacked in small orbits around a red dwarf star not much bigger than Jupiter. Remarkably, all seven of the planets have size and density similar to Earth’s (see Fig. IV.7) and four of them may lie in the habitable zone. The average density of the TRAPPIST-1 planets is about 8% less than that of Earth, leading astrophysicists to offer the possible models of their planetary composition in Fig. IV.8.
NASA was sufficiently excited by the TRAPPIST-1 system to commission a “planet-hopping” travel poster (Fig. IV.9) focused on the planet-filled view of the night sky from TRAPPIST-1e, with its size and density close to Earth’s, its location smack in the middle of the habitable zone and of its planetary system, its possibly rocky and watery surface, its gravity not much weaker than Earth’s, and its remarkable 6-day long complete orbit around its local star. And the TRAPPIST-1 system is not even the closest discovered exoplanetary system to Earth. That distinction falls to the three planets found so far orbiting Proxima Centauri, only 4.2 light-years from Earth. That system contains at least two planets within the habitable zone. But before we get ahead of ourselves, let’s remember that residence within the habitable zone is a necessary, but not sufficient, condition for life to be supported. A habitable planet must also have a suitable atmosphere, and there is considerable question (see below) whether planets around M dwarf stars, such as TRAPPIST-1 and Proxima Centauri, can support appropriate atmospheres. Probes of their atmospheres are near-time goals of the exoplanet strategy.
V. next stages of exoplanet exploration
Exoplanet research is now a mature field, poised to transition from the discovery phase to the characterization phase. The strategy for exoplanet science over the coming decades was laid out in a 2018 National Academies report. The needs specifically in the search for other Earths include: broadening the search space to include wider patches of the Milky Way and greater sensitivity to solar system-like planets; measuring the density of more identified exoplanets to find potentially rocky planets in habitable zones; carrying out atmospheric spectroscopy on promising candidates, especially ones in relatively close proximity to Earth; and enhancing understanding of potential biosignatures of life on exoplanets, and conditions that might yield false positive or false negative signals. In addition to these goals, there continues to be great interest in the worldwide astrophysics community in extending study of gas giants and other non-Earth-like planets, in order to illuminate the processes by which planets form and evolve.
As demonstrated vividly by the enormous progress made in the preceding decade, advancing research relies on advancing the technology of space telescopes. Figure V.1 shows the past, present and future space telescopes deployed and considered for exoplanet research by both NASA and the European Space Agency (ESA). As the sophistication and sensitivity of these telescopes advances, so does the price tag. The recently launched James Webb Space Telescope (JWST) cost about $10 billion U.S. to construct. At such price tags, these telescopes must serve a broader array of scientific goals than just exoplanet research. Those currently in service and planned for the future are also designed to peer much deeper into space beyond the Milky Way, to probe the early stages of the universe, the formation of the first stars and galaxies, and the nature of dark energy.
The newest space telescopes already deployed are the JWST and the ESA’s CHEOPS (CHaracterizing ExOPlanet Satellite). CHEOPS was launched in December 2019, with the primary goal of making more precise size measurements, enabling density determinations, for exoplanets in the super-Earth to Neptune size range transiting in front of bright, nearby stars.
The JWST is currently in its commissioning phase in orbit around the Lagrange point L2 (see Fig. III.10). The mission is planned, at least initially, for ten years of service, expected by the end to exhaust the fuel needed to orient the mirrors for focus on particular stars of interest. During that period of service, with its onboard coronagraphs to shield direct light from the stars, it should “provide infrared spectra of terrestrial-size planets in and near the liquid water habitable zones of M dwarfs, as well as a statistical sample of spectra for up to about 100 larger and hotter planets. The latter will enable the search for physical and chemical trends across objects, while the former will significantly advance understanding of planetary habitability. The spectra of warm terrestrials will answer the critical question of what kinds of atmospheres rocky planets around M dwarfs can form and retain. In terms of potentially habitable worlds, JWST may have the capability to detect molecules (such as H2O, CO2, and CH4, although most likely not O2) in the atmospheres of a handful of the most favorable planets.” As seen in Fig. III.7, the best signatures of oxygen and ozone molecules in the atmosphere are sharp absorption dips in the visible light range, outside JWST’s mid-infrared sensitivity.
Whether rocky exoplanets around M dwarfs, like those in the TRAPPIST-1 system (Fig. IV.7), are capable of forming and retaining atmospheres and life is a critical question in the search for habitability. Habitable zone planets orbit so close to these dwarf stars that they may potentially be tidally locked, with one side always facing the star and the other always facing away. If that is the case, the permanently cold, dark side of such planets may freeze out any atmosphere that tries to form. Furthermore, dwarf stars are generally more violent and variable than Sun-like stars. They sometimes emit giant flares that can double their brightness in a matter of minutes, and it may be difficult for life to adapt to such extreme variability. Even when stable, their emissions are primarily in the red and infrared portions of the spectrum, as opposed to the UV-rich spectrum from Sun-like stars. The lower-energy infrared emissions make photosynthesis much more challenging and are strongly absorbed in water, making less energy available for underwater life. The infrared spectra measured by JWST will help to address these issues.
Two already approved missions aiming for launches later in this decade are ESA’s PLATO (PLAnetary Transits and Oscillations of stars) and NASA’s NGRST (previously WFIRST). The main focus of PLATO is the search, among up to a million stars, for Earth-like planets in the habitable zone transiting Sun-like stars. A secondary objective is to provide precise characterization of the mass and evolution stage of the planets’ host stars, by studying stellar oscillations and seismic activity. It is scheduled for launch in 2026.
The NGRST, aiming for launch by 2027, will carry two instruments with sensitivity in the visible and near infrared portions of the electromagnetic spectrum. The wide field instrument should provide resolution similar to that achieved with the Hubble Space Telescope, but with 100 times larger field of view. In addition to providing deep space probes of cosmic structures, it will revolutionize the use of gravitational microlensing for identification and mass determination of exoplanets. As seen in Fig. V.2, this microlensing survey will go far beyond the orbital period limits of previous transiting surveys, so that it should be able to reveal other planetary systems with greater similarity to the large orbits around Sun-like stars of our solar system.
The second NGRST instrument is a high-contrast, narrow-field coronagraph, whose blocking of direct starlight should facilitate the direct imaging and reflected light spectroscopy of dozens of individual nearby exoplanets, including those in the TRAPPIST-1 system, for comparison to that of Earth in Fig. III.7. The direct imaging and spectroscopic studies will be complemented by two giant new ground-based telescopes planned for implementation toward the end of the present decade. The Giant Magellan Telescope and the Thirty Meter Telescope will provide a “3-fold improvement in angular resolution, 10-fold improvement in light-collecting capabilities, and 80-fold improvement in sensitivity to point sources” over the previous generation of ground-based telescopes. Crucially, these ground-based telescopes and the NGRST coronagraph instrument will complement JWST in being able to detect oxygen and ozone absorption lines – so important as biosignatures – in the visible light spectrum.
For the longer term, the Exoplanet Science Strategy has identified “a large space-based, direct imaging mission, capable of directly detecting and characterizing terrestrial planets in reflected light around Sun-like stars at near-ultraviolet, optical, and near-infrared wavelengths, to be the primary long-term priority for NASA exoplanet science. Such a mission would explore the atmospheres of planets with a range of sizes and effective temperatures, and image multiple planets in each system, enabling comparative exoplanetology. A direct imaging mission would also be sensitive to terrestrial planets in the habitable zones of stars similar to the Sun, environments for which Earth provides a proof of concept that habitability is possible, but which are accessible only from space.” This recommendation was endorsed for major future priority by the 2021 National Academies Decadal Survey of Astronomy and Astrophysics.
Those goals are embodied in the New Worlds Mission under present consideration. Possible implementations discussed in the Decadal Survey include the Habitable Exoplanet Observatory (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR), each presently judged to be a roughly $10 billion U.S. project. Either instrument would have a primary focus on searching for signs of habitability and potential biosignatures over a broad spectral range in the atmospheres of Earth-sized habitable zone planets orbiting nearby main sequence stars. Examples of such planets already known are shown in Fig. V.3. The direct imaging would be enhanced by either next-generation coronagraphs, learning from experience with the NGRST, or a starshade, like that pictured in Fig. III.6, if the technology advances rapidly enough. Either mission would be unlikely to fly before the late 2030s.
By the middle of this century, then, upcoming telescopes should have collected reflected light spectra for a significant number of the best habitable zone exoplanet candidates. The search for biosignatures will certainly be guided by the features noted in the Earthshine spectrum of Fig. III.7 (reproduced here as Fig. V.4 for convenience). But there is currently lots of modeling research in progress attempting to understand how one or more of those features might disappear even on a planet actually capable of hosting life (false negative biosignatures) or might appear without actually signaling life hospitability (false positives). As suggested by simulation results reflected in Fig. V.5, a true positive biosignature might require simultaneous observation of absorption lines corresponding to oxygen, ozone, carbon dioxide and methane in the atmosphere. But models have been able to produce at least two of those molecules even for inhospitable planets that are covered completely by deserts, or are completely water-logged, or are surrounded by an atmosphere that causes an eternal runaway greenhouse effect.
Shawn Domagal-Goldman of NASA’s Goddard Space Flight Center has explained why the combination of oxygen and methane molecules in an exoplanet atmosphere is a particularly promising biosignature. Methane (CH4) can be produced by bacteria or in cow flatulence, as it is on Earth, but also from non-biological sources like underwater volcanic eruptions. Oxygen and ozone can be produced as a result of photosynthesis and UV irradiation, as they are on Earth, but also without life when UV light breaks up carbon dioxide molecules in the atmosphere. But methane doesn’t last for very long in an atmosphere containing other molecules with oxygen atoms.
Thus, according to Domagal-Goldman, “It’s like college students and pizza. If you see pizza in a room, and there are also college students in that room, chances are the pizza was freshly delivered, because the students will quickly eat the pizza. The same goes for methane and oxygen. If both are seen together in an atmosphere, the methane was freshly delivered because the oxygen will be part of a network of reactions that will consume the methane. You know the methane is being replenished. The best way to replenish methane in the presence of oxygen is with life. The opposite is true, as well. In order to keep the oxygen around in an atmosphere that has a lot of methane, you have to replenish the oxygen, and the best way to do that is with life.”
In addition to atmospheric absorption lines, model simulations carried out by astrobiology teams suggest that it may be important to look for reflected light signatures of plant life on a planet’s surface. Potential signatures include not merely evidence of the sort of “vegetation jump” seen in Earthshine (Fig. V.4), but possible seasonal variations in the extent of that jump, if the planet has a long enough orbit period (and a tilted axis, like Earth) to detect seasons.
The enormous progress of recent decades in identifying exoplanets, and the anticipation of a continuing acceleration in the pace of discovery in the coming decades, have led to more detailed considerations of all the factors that may impact a planet’s habitability. As indicated in Fig. V.6, it is not just a matter of a planet’s size, orbit and atmosphere; habitability can be influenced by the detailed structure of its surface and interior. Not only the size, age and the spectral distribution of light emitted by the star it orbits, but also the structure of the star and its location within a galaxy, may affect the planet’s ability to sustain surface liquid water. That ability can also potentially be influenced by the presence of other planets orbiting the same star. There are so many factors that may possibly have impact that it may seem fortuitous that even Earth manages to support life. But, given the implication from discoveries so far that at least several billion other Earth-like planets orbit Sun-like stars within the habitable zone within the Milky Way galaxy alone, it is unlikely that Earth is unique.
VI. brief summary
More than 5000 exoplanets have been discovered to date, via a number of different techniques, nearly all looking at the indirect effects an orbiting planet has on the light received on Earth or in a space telescope from the star the planet orbits. Roughly 4% of these appear from measurements of size, mass, density and orbit period to be rocky planets of size similar to Earth, orbiting at distances from their stars at which it is conceivable to sustain liquid surface oceans. Some of these terrestrial planets are not too distant from Earth, relative to cosmic distance scales. And some dwell in intriguing non-solar planetary systems.
Newly launched and anticipated future telescope missions advancing the technical state-of-the-art will not only greatly expand this pool of candidates for habitability, but will begin to provide detailed measurements of the spectrum of light reflected or emitted from those planets, or absorbed in their atmosphere when they transit in front of their local stars. Those spectroscopic measurements, and the chemical composition of exoplanet atmospheres they reveal, should provide first hints of possible life elsewhere in our galaxy.
But the search for other Earths cannot, on its own, provide definitive proof that life exists elsewhere in the universe. What it will do is narrow down the pool of planet candidates to the most promising places to look for more direct observation of living organisms or contact with other beings. The latter possibility is the subject of Part III.
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