For countless generations, human beings have looked out at the night sky and wondered if they were alone in the Universe. With the discovery of other planets in our Solar System, the true extent of the Milky Way galaxy, and other galaxies beyond our own, this question has only deepened and become more profound.
And whereas astronomers and scientists have long suspected that other star systems in our galaxy and the Universe had orbiting planets of their own, it has only been within the last few decades that any have been observed. Over time, the methods for detecting these “extrasolar planets” have improved, and the list of those whose existence has been confirmed has grown accordingly (over 4000 and counting!)
An extrasolar planet (aka. exoplanet) is a planet that orbits a star (i.e. is part of a solar system) other than our own. Our Solar System is only one among billions and many of them most likely have their own system of planets. As early as the sixteenth century, there have been astronomers who hypothesized the existence of extrasolar planets.
The first recorded mention was made by Italian philosopher Giordano Bruno, an early supporter of the Copernican theory. In addition to supporting the idea that the Earth and other planets orbit the Sun (heliocentrism), he put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets.
In the eighteenth century, Isaac Newton made a similar suggestion in the “General Scholium” section which concludes his Principia. Making a comparison to the Sun’s planets, he wrote “And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One.”
Since Newton’s time, various discovery claims have been made, but all were rejected by the scientific community as false positives. In the 1980s, a group of astronomers claimed that they had identified some extrasolar planets in nearby star systems, but were unable to confirm their existence until years later.
One of the reasons why extrasolar planets are so difficult to detect is because they are even fainter than the stars they orbit. Additionally, these stars give off light that “washes” the planets out – i.e. obscures them from direct observation. As a result, the first discovery was not made until 1992 by astronomers Aleksander Wolszczan and Dale Frail.
Using the Arecibo Observatory in Puerto Rico, the pair observed several terrestrial-mass planets orbiting the pulsar PSR B1257+12. It was not until 1995 that the first exoplanet confirmation around a main-sequence star was made. In this case, the planet observed was 51 Pegasi b, a giant planet found in a four-day orbit around the Sun-like star 51 Pegasi (approx 51 light-years from our Sun).
Initially, most of the planets detected were gas giants similar to, or larger than, Jupiter – which led to the term “Super-Jupiter” being coined. Far from suggesting that gas giants were more common than rocky (i.e. “Earth-like“) planets, these findings were simply due to the fact that Jupiter-sized planets are simply easier to detect because of their size.
The Kepler Mission:
Named after the Renaissance astronomer Johannes Kepler, the Kepler space observatory was launched by NASA on March 7th, 2009 for the purpose of discovering Earth-like planets orbiting other stars. As part of NASA’s Discovery Program, a series of relatively low-cost projects focused on scientific research, Kepler‘s mission was to find evidence of extrasolar planets and estimate how many stars in our galaxy have planetary systems.
Relying on the Transit Method of detection (see below), Kepler‘s sole used a photometer to continually monitor the brightness of over 145,000 main sequence stars in a fixed field of view. This data was then transmitted back to Earth where it was analyzed by scientists to look for any signs of periodic dimming caused by extrasolar planets transiting (passing) in front of their host star.
The initial planned lifetime of the Kepler mission was 3.5 years, but greater-than-expected results led to the mission being extended. In 2012, the mission was expected to last until 2016, but this changed due to the failure of two of the spacecraft’s reaction wheels – which are used for pointing the spacecraft. This disabled the collection of science data and threatened the continuation of the mission.
On August 15th, 2013, NASA announced that they had given up trying to fix the two failed reaction wheels and modified the mission accordingly. Rather than scrap Kepler, NASA proposed changing the mission to utilizing Kepler to detect habitable planets around smaller, dimmer red dwarf stars. This proposal, which became known as K2 “Second Light“, was approved on May 16th, 2014.
The K2 mission (which lasted until ) focused more on brighter stars (such as G- and K-class stars). As of February 6th, 2021, astronomers have confirmed the presence of 4,341 exoplanets in 3,216 planetary systems, the majority of which were found using data from Kepler. All told, the space probe observed over 530,506 stars in the course of its primary and K2 missions.
In November of 2013, astronomers reported (based on Kepler space mission data) that 1 in 5 stars in the Milky way could have Earth-sized planets orbiting within their habitable zones – between 40 and 80 billion. They further estimated that 7 to 15% of these planets (average of 5.6 billion) orbit Sun-like stars – aka. main sequence G-type yellow dwarfs.
The first exoplanet confirmed by Kepler to have an average orbital distance that placed it within its star’s habitable zone was Kepler-22b. This planet is located about 600 light-years from Earth in the constellation of Cygnus and was first observed on May 12th, 2009, and then confirmed on Dec 5th, 2011. Based on all the data obtained, scientists believe that this world is roughly 2.4 times the radius of Earth and either has oceans or a watery outer shell.
The discovery of exoplanets has also intensified interest in the search for extraterrestrial life, particularly for those that orbit in the host star’s habitable zone. Also known as the “goldilocks zone“, this is the region of the solar system where conditions are warm enough (but not too warm) so that it is possible for liquid water (and therefore life) to exist on the planet’s surface.
Prior to the deployment of Kepler, the vast majority of confirmed exoplanets fell into the category of Jupiter-sized or larger. However, over the course of its missions, Kepler managed to identify over 6000 potential candidates, many of them falling into the categories of Earth-size or “Super-Earth” size. Many of these are located in the habitable zone of their parent stars, and some even around Sun-like stars.
And according to a study conducted by NASA’s Ames Research Center, analysis of the Kepler mission data indicated that about 24% of M-class stars may harbor potentially habitable, Earth-size planets (i.e. those that are smaller than 1.6 times the radius of Earth’s). Based upon the number of M-class stars in the galaxy, that alone represents about 10 billion potentially habitable, Earth-like worlds.
Meanwhile, analyses of the K2 phase suggest that about one-quarter of the larger stars surveyed may also have an Earth-size planet orbiting within their habitable zones. Taken together, the stars observed by Kepler make up about 70% of those found within the Milky Way. So one can estimate that there are literally tens of billions of potentially habitable planets in our galaxy alone.
While some exoplanets have been observed directly with telescopes (a process known as “Direct Imaging“), the vast majority have been detected through indirect methods such as the transit method and the radial-velocity method. In the case of the Transit Method (aka. Transit Photometry), a planet is observed when crossing the path (i.e. transiting) in front of its parent star’s disk.
When this occurs, the observed brightness of the star drops by a small amount. This can be used to determine the radius of the planet and can sometimes allow a planet’s atmosphere to be investigated through spectroscopy. However, it also suffers from a substantial rate of false positives and requires that part of the planet’s orbit intersects with a line-of-sight between the host star and Earth.
As a result, confirmation from another method is usually considered necessary. Nevertheless, it remains the most widely used method and is responsible for more exoplanet discoveries than all other methods combined. Both the Kepler Space Telescope and TESS were specifically designed to conduct this kind of photometry (see above).
The Radial Velocity (or Doppler Method) involves measuring the star’s radial velocity – i.e. the speed with which it moves towards or away from Earth. The is a means of detecting planets because, as planets orbit a star, they exert a gravitational influence that causes the star itself to move in its own small orbit around the system’s center of mass. This method has the advantage of being applicable to stars with a wide range of characteristics.
However, one of its disadvantages is that it cannot determine a planet’s true mass, but can only set a lower limit on that mass. It remains the second-most effective technique employed by exoplanet hunters. Other methods include Transit Timing Variation (TTV) and Gravitational Microlensing. The former relies on measuring the variations in the times of transit for one planet to determine the existence of others.
This method is effective in determining the existence of multiple transiting planets in one system but requires that the existence of at least one already be confirmed. In another form of the method, timing the eclipses in an eclipsing binary star can reveal an outer planet that orbits both stars. As of February 2020, 21 planets have been found with this method while numerous more were confirmed.
In the case of Gravitational Microlensing, this refers to the effect a star’s gravitational field can have, acting as a lens to magnify the light of a distant background star. Planets orbiting this star can cause detectable anomalies in the magnification over time, thus indicating their presence. This technique is effective in detecting stars that have wider orbits (1-10 AUs) from Sun-like stars.
Other methods exist, and – alone or in combination – have allowed for the detection and confirmation of over four thousand exoplanets, while another 5,742 candidates await confirmation. Of these, 1473 (34%) have been gas giants comparable to Neptune (Neptune-like), while 1359 (31%) have been gas giants comparable to Jupiter (Jupiter-like).
Another 1340 (31%) have been terrestrial planets that are several times more massive than Earth (Super-Earths) while 163 have been comparable to Earth in terms of size and mass (4%). A further 6 exoplanets have been detected and confirmed that remain unclassified.
Closest to Earth
On August 24th, 2016, the ESO confirmed the existence of an Earth-sized rocky exoplanet orbiting Proxima Centauri, an M-type (red dwarf) star located 4.25 light-years away. This makes this particular exoplanet, known as Proxima b, is the closest exoplanet to Earth. Equally important is the fact that it is believed to orbit within Proxima Centauri’s habitable zone.
The discovery was made by the Pale Red Dot campaign and a team of astronomers led by Dr. Guillem Anglada-Escudé of the Queen Mary University of London. Based on observations made using the High Accuracy Radial Velocity Planet Searcher (HARPS) and Ultraviolet and Visual Echelle (UVE) spectrographs at the ESO’s La Silla Observatory and Very Large Telescope.
Based on the data obtained by the Pale Red Dot campaign and subsequent observations, Proxima b is estimated to be 1.2 times as massive as Earth and between one and 1.3 times its size. It orbits its parent star at a distance of roughly 0.05 AU (7.5 million km; 4.6 million) and takes just 11.2 days to complete a single orbit. Like many rocky planets orbiting M-type stars, Proxima b is believed to be tidally-locked.
Given the tenuous nature of M-type stars and their tendency to produce powerful flares, it is unclear whether or not Proxima b could maintain an atmosphere and liquid water on its surface over time. Multiple studies and climate models have been performed to determine the likelihood of Proxima b being able to support life, but no scientific consensus has emerged.
On the one hand, multiple studies have concluded that solar flare activity from its host star would inevitably strip Proxima b of its atmosphere and irradiate the surface. Meanwhile, other research and modeling have found that if Proxima b has a magnetic field, a dense atmosphere, and plenty of surface water and cloud cover, the odds of it being habitable are encouraging.
In January of 2020, an INAF-led team of astronomers announced the possible detection of a second planet around Proxima Centauri (using Radial Velocity measurements). According to the research team’s paper, their measurements indicated the presence of a mini-Neptune (Proxima c) orbiting its parent star at a distance of 1.5 AU (~224.4 million km; ~139.4 million mi).
By June of 2020, a team of astronomers from the University of Texas’ McDonald Observatory used radial velocity measurements gathered by Hubble (25 years ago) to confirm the presence of Proxima c. Their research also placed tighter constraints on the planet’s mass and orbital period, which are now estimated at 0.8 Jupiter masses and ~1900 days, respectively.
In December of 2020, astronomers at the Parkes radio telescope in Australia announced the detection of a “tantalizing” radio signal coming from the direction of Proxima Centauri. The signal was picked up between April and May of 2019 as part of a Breakthrough Listen observation campaign. This signal, Breakthrough Listen Candidate 1 (BLC1), lasted for 30 hours and showed a number of curious features.
For instance, the signal was an extremely sharp narrowband emission – at 982 megahertz (MHz) – that appeared to be undergoing a shift in frequency (aka. Doppler shift). According to various astrophysicists, this is consistent with a moving source (i.e. a planet orbiting its star). However, the scientific community has since announced that the signal is unlikely to be anything other than the result of natural phenomena.
On April 18th, 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS) to space. This mission has effectively picked up the trail blazed by Kepler, using the same method but superior instruments to monitor thousands of stars simultaneously. Equipped with four wide-angle telescopes and associated charge-coupled device (CCD) detectors, TESS is currently carrying the first spaceborne all-sky transiting exoplanet survey.
TESS’s primary mission lasted two years – officially ending on July 5th, 2020 – followed by NASA announcing a 27-month extension on August 12th. For the first year of its Extended Mission, TESS will re-observe the southern ecliptic hemisphere (which it monitored during its primary mission) and the next 15 months monitoring art of the northern ecliptic hemisphere and ~60% of the ecliptic.
During its primary mission, TESS scanned about 75% of the sky and surveyed 200,000 of the brightest stars near the Sun for signs of transiting exoplanets. As of February 6th, 2021, the TESS mission has detected a total of 2,487 exoplanets and confirmed 107, ranging from terrestrial candidates to super-Jupiters.
In addition, the European Space Agency’s (ESA) Gaia Observatory continued to monitor the precise positions, proper motions, and orbits of over 1 billion stars, planets, comets, asteroids and quasars. This mission commenced operations in 2013 (the same year that the ESA’s Herschel Space Telescope retired) and its primary mission was intended to last five years.
Currently, Gaia is in an extended part of its mission that will last until December 31st, 2022, though it is expected to receive another extension to December 31st, 2025. To date, the mission has been in continuous operation for 7 years, 1 month, and 18 days, and will continue to map the cosmos for the sake of creating the largest and most precise 3D space catalog ever made.
Another exoplanet-hunting mission overseen by the ESA is the CHaracterising ExOPlanets Satellite (CHEOPS), which launched on Dec. 18th, 2019, and is the first Small-class mission in the ESA’s Cosmic Vision science program. Between now and the end of its primary mission (scheduled for mid-2023), CHEOPS will study known exoplanets to obtain more accurate estimates on their mass, density, composition, and formation.
And of course, there’s the venerable Hubble Space Telescope, which has remained in operation for over 30 years! In addition to making profound discoveries that have altered our perception of the Universe around us (such as measuring the rate of cosmic expansion, leading to the theory of Dark Energy), Hubble has also played a vital role in the detection and characterization of exoplanets.
For instance, early in its mission, Hubble detected debris disks around distant stars (from which planets form) as well as planetary systems that were in the process of formation. Meanwhile, the archives of Hubble’s past observations have allowed astronomers to go back and find evidence of planets making transits in front of their stars, as well as provide spectra that allowed for the characterization of exoplanet atmospheres.
Hubble’s many years of observation also helped astronomers to learn about the diversity of exoplanets and establish the current method for classifying them. On top of all that, Hubble has taught astronomers a great deal about the diversity of parent stars and how their characteristics can influence a planet’s habitability.
In the coming years, several next-generation space telescopes will be sent to space to aid in the ongoing hunt for habitable exoplanets. On October 31st, 2021, NASA’s long-awaited James Webb Space Telescope (JWST) will be launched to its position at the Sun-Earth L2 Lagrange Point. This mission will be the largest and most-sophisticated space telescope to date and will have to go through a complex deployment phase once it’s in position.
Using its highly-sophisticated infrared (IR) suite and light-blocking coronographs, the JWST will be able to detect lower-mass exoplanets that orbit nearer to their stars. This is where most Earth-like rocky planets that orbit within a star’s habitable zone (and are therefore considered to be “potentially-habitable”) are expected to be found.
Until now, existing space telescopes do not have the resolution or sensitivity to study these planets via Direct Imaging. Existing telescopes have also not been able to obtain spectra from smaller, rocky planets when they transit in front of their stars. However, the JWST instruments will be able to determine the chemical composition of exoplanet atmospheres by examining which IR wavelengths are absorbed and/or radiated.
There’s also the Nancy Grace Roman Space Telescope, a successor mission nicknamed the “Mother of Hubble.” Combing a 2.4 meter (ft) primary mirror with the Wide-Field Instrument IR camera, a coronograph, a spectrometer, and a large field of view, the Roman space telescope will be able to bring the same image sharpness of Hubble to an area of the sky 100 times as large.
The ESA is also prepping a series of next-generation observatories, like the PLAnetary Transits and Oscillations of stars (PLATO) space telescope. This mission will observe up to one million stars for planetary transits, attempt to characterize their atmospheres, and characterize stars by measuring their oscillations. This will be the third medium-class mission in the ESA’s Cosmic Vision program and is scheduled to launch sometime in 2022.
This will be followed by the Cosmic Vision’s fourth-medium mission, known as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL). This mission, which will launch sometime in 2029, will observe at least 1,000 known exoplanets as they transit in front of their stars to study and characterize the composition and thermal structures of their atmospheres.
There’s an entire Universe of worlds out there to discover, and we’ve barely scratched the surface!
Universe Today has many interesting articles on exoplanets. Here’s What Does “Earthlike” Even Mean & Should It Apply To Proxima Centauri b?, Focusing On ‘Second-Earth’ Candidates In The Kepler Catalog, New Technique to Find Earth-like Exoplanets, Potentially Habitable Exoplanet Confirmed Around Nearest Star!, Planetary Habitability Index Proposes A Less “Earth-Centric” View In Search Of Life, Habitable Earth-Like Exoplanets Might Be Closer Than We Think.
For more information, check out Kepler‘s home page at NASA. The Planetary Society’s page on Exoplanets is also interesting, as is the NASA Exoplanet Archive – which is maintained with the help of Caltech.
Astronomy Cast has an episode on the subject – Episode 2: In Search of Other Worlds.