For endless generations, people have wondered if they are alone in the universe as they gaze up at the night sky. This question has only deepened and deepened with the discovery of additional planets in our solar system, the true extent of the Milky Way galaxy, and other galaxies outside of our own.
While astronomers and scientists have long speculated that other star systems in our galaxy and universe have their orbiting planets, none have been discovered until the last few decades. The technologies for identifying these “extrasolar planets” have improved over time, and the number of those whose existence has been proven has also increased (nearly 4000 and counting!).
A planet that orbits a star other than our own (i.e. is part of a solar system) is called an extrasolar planet (or exoplanet). Our solar system is one of billions, and many of them probably have their own planetary systems. Astronomers have speculated about the existence of extrasolar planets since the 16th century.
The following is a list of potentially habitable exoplanets discovered in our universe so far. courtesy of phl.upl.edu
The Italian philosopher Giordano Bruno, an early supporter of the Copernican idea, provided the first documented mention. He proposed that fixed stars are analogous to the sun and are accompanied by planets, in addition to supporting the theory that the earth and other planets orbit the sun (heliocentrism).
Isaac Newton made a similar remark in the General Scholium section of his Principia in the 18th century. “And if the fixed stars are the centers of comparable systems, they will all be formed of a similar design, and subject to the Rule of One,” he wrote, making a connection to the planets of the sun.
Various discovery claims have been made since Newton’s time, but they have always been dismissed as false positives by the scientific community. A group of astronomers claimed to have discovered extrasolar planets in nearby star systems in the 1980s, but were only able to confirm their existence years later.
Extrasolar planets are difficult to identify because they are even fainter than the stars they orbit. In addition, these stars emit light that “washes out” the planets, obscuring them from direct view. As a result, astronomers Aleksander Wolszczan and Dale Frail didn’t make the first discovery until 1992.
The pair observed five terrestrial-mass planets orbiting pulsar PSR B1257+12 using the Arecibo Observatory in Puerto Rico. The first confirmation of an exoplanet orbiting a main sequence star was not made until 1995. 51 Pegasi b, a giant planet discovered in a four-day orbit around the sun-like star 51 Pegasi, was the planet seen in this case (about 51 light-years from our sun).
The majority of planets first discovered were gas giants similar to or larger than Jupiter, leading to the coining of the nickname “Super Jupiter”. These results did not imply that gas giants outnumbered rocky (i.e., “Earth-like”) planets; Rather, Jupiter-sized planets are simply easier to spot because of their size.
The Kepler Project:
The Kepler Space Observatory, named after Renaissance astronomer Johannes Kepler, was launched by NASA on March 7, 2009 with the goal of identifying Earth-like planets orbiting other stars. Kepler’s goal was to find evidence of extrasolar planets and determine how many stars in our galaxy have planetary systems as part of NASA’s Discovery program, a series of relatively inexpensive programs focused on scientific study.
Kepler’s sole used a photometer to continuously monitor the brightness of over 145,000 main sequence stars in a fixed field of view, relying on the transit detection method (see below). This information was then sent back to Earth, where scientists examined it for signs of a periodic eclipse caused by extrasolar planets passing (passing) in front of their host star.
The Kepler mission was originally scheduled to last 3.5 years but was extended due to better than expected results. The mission was originally scheduled to last until 2016, but was canceled due to the loss of two spacecraft reaction wheels responsible for the spacecraft’s orientation. This stopped collecting scientific onesdata and jeopardized the survival of the mission.
NASA announced on August 15, 2013 that it had given up trying to replace the two broken reaction wheels and changed the mission accordingly. Rather than abolish Kepler, NASA recommended repurposing it to look for habitable planets around smaller, fainter red dwarf stars. On May 16, 2014, this plan, known as K2 “Second Light”, was approved.
The K2 expedition (which lasted to ) dealt mostly with brighter stars (such as G- and K-class stars). Astronomers have confirmed the presence of 4,341 exoplanets in 3,216 planetary systems as of February 6, 2021, the majority of which were discovered using Kepler data. During its primary and K2 missions, the spacecraft saw a total of 530,506 stars.
Astronomers announced in November 2013 that 1 in 5 stars in the Milky Way could contain Earth-sized planets orbiting within their habitable zones, based on data from the Kepler Space Project. They also predicted that 7 to 15% of these planets (an average of 5.6 billion) orbit Sun-like stars, also known as main-sequence G-type yellow dwarfs.
The distances of the planets of the Solar System (top row) and the Gliese 581 system (bottom row) from their respective stars are shown in this diagram (left). The habitable zone is shown in blue, indicating that Gliese 581 d is within the habitable zone surrounding its low-mass red star. Based on a diagram by Franck Selsis of the University of Bordeaux. ESO is responsible for this image.
Based on the study by Franck Selsis of the University of Bordeaux, this diagram shows the habitable zone of the solar system (top row) and the Gliese 581 system (bottom row). ESO is responsible for this image.
Kepler-22b was the first exoplanet discovered by Kepler, with an average orbital distance that placed it within its star’s habitable zone. Located approximately 600 light-years from Earth, this planet in the constellation Cygnus was discovered on May 12, 2009 and confirmed on December 5, 2011. Scientists estimate that this world is about 2.4 times the radius of Earth and has either oceans or a watery outer shell based on all available data.
The discovery of exoplanets has sparked interest in searching for extraterrestrial life, particularly those orbiting in the habitable zone of their host star. In this region of the solar system, often referred to as the “Goldilocks Zone,” conditions are warm enough (but not too enough) for liquid water (and thus life) to exist on a planet’s surface.
Prior to Kepler’s launch, the vast majority of verified extrasolar planets were Jupiter-sized or larger. Kepler, on the other hand, identified over 6,000 possible candidates over the course of his missions, many of which fit into the Earth or “super-Earth” size classification. Many of them are in the habitable zones of their host stars, and some even orbit sun-like stars.
According to research from NASA’s Ames Research Center, about 24 percent of M-class stars have potentially habitable, Earth-sized planets (those with a radius less than 1.6 times Earth’s radius). Based on the number of M-class stars in the galaxy, this equates to approximately 10 billion potentially habitable Earth-like worlds.
Meanwhile, K2 phase surveys suggest that about a quarter of the larger stars surveyed may have an Earth-sized planet orbiting within their habitable zones. The stars discovered by Kepler make up about 70% of all stars in the Milky Way. As a result, tens of billions of potentially habitable planets can be estimated in our galaxy alone.
While some exoplanets have been discovered directly using telescopes (known as “direct imaging”), the vast majority have been discovered through indirect methods such as transit and radial velocity approaches. A planet is detected using the transit method (also known as transit photometry) when it crosses the path (i.e. transits) in front of its parent star’s disk.
When this happens, the star’s brilliance is reduced by a modest amount. This can be used to calculate planetary radius, and it can also be used to analyze a planet’s atmosphere using spectroscopy. However, it carries a high risk of false alarms and requires part of the planet’s orbit to intersect a line of sight between the host star and Earth.
As a result, confirmation from another agent is often required. Nonetheless, it’s still the onemost widely used approach, with more exoplanet discoveries than all other methods combined. Both the Kepler space telescope and TESS were built with this type of photometry in mind (see above).
Radial velocity (or Doppler method) involves determining the star’s radial velocity, or how fast it is moving toward or away from Earth. Planets orbiting a star exert a gravitational effect that causes the star to move in its own small orbit around the system’s center of mass, which can be used to detect planets. This method has the advantage that it can be adapted to stars with a wide range of properties.
However, one of its disadvantages is that it can only set a lower bound on a planet’s true mass, rather than determine it. Exoplanet hunters still use it as the second most effective method. Transit timing variation (TTV) and gravitational microlensing are two other techniques. The former focuses on determining the existence of other planets by measuring differences in transit times for one.
This method is useful for determining the existence of numerous passing planets in a single system, but it requires confirmation of at least one. The timing of eclipses in an eclipsing binary can, in another version of the technique, reveal an outer planet orbiting both stars. Using this approach, 21 planets have been discovered as of February 2020, and many more have been verified.
The effect of a star’s gravitational field acting as a lens to magnify the light of a distant background star is known as gravitational microlensing. Planets orbiting this star can produce magnification anomalies that can be detected over time, confirming their presence. This method works well for finding stars with larger orbits (1-10 AU) than Sun-like stars.
There are other approaches that have led to the discovery and confirmation of nearly 4,000 exoplanets, with another 5,742 candidates awaiting confirmation. Of these, 1473 (34 percent) were Neptune-like gas giants (Neptune-like) and 1359 (31 percent) were Jupiter-like gas giants (Jupiter-like).
Another 1,340 (31%) were terrestrial planets many times more massive than Earth (super-earths), while 163 were similar in size and mass to Earth (4 percent). A total of 6 unidentified exoplanets have been discovered and verified.
Closest to Earth
ESO on 24 August 2016 confirmed the existence of an Earth-sized rocky exoplanet orbiting Proxima Centauri, a type M (red dwarf) star 4.25 light-years away from the nearest exoplanet to Earth. The fact that it is believed to orbit within the habitable zone of Proxima Centauri is also significant.
The Pale Red Dot campaign and a team of astronomers led by Dr. Guillem Anglada-Escudé of Queen Mary University of London made the discovery. Based on observations made at La Silla Observatory and ESO’s Very Large Telescope with the High Accuracy Radial Velocity Planet Searcher (HARPS) and the Ultraviolet and Visual Echelle (UV) spectrographs.
Proxima b is believed to be 1.2 times Earth’s massive and between 1 and 1.3 times larger based on data from the Pale Red Dot study and subsequent observations. It orbits its parent star at a distance of about 0.05 AU (7.5 million km; 4.6 million) and completes one orbit in about 11.2 days. Like many rocky planets orbiting M-type stars, Proxima b is believed to be tidally trapped.
It is unknown whether Proxima b could sustain an atmosphere and liquid water on its surface over time, given the fragile nature of M-type stars and their propensity to produce massive flares. Several analyzes and climate models have been performed to estimate the likelihood of Proxima b supporting life, but no scientific consensus has emerged.
On the one hand, several studies concluded that solar flare activity from Proxima b’s host star would inevitably strip the planet of its atmosphere and irradiate the surface. Meanwhile, other research and models suggest that if Proxima b has a magnetic field, dense atmosphere, and plenty of surface water and cloud cover, it has a good chance of being habitable.
In January 2020, an astronomical team led by the INAF announced the probable discovery of a second planet around Proxima Centauri (using radial velocitiesits measurements) known. According to the research team’s findings, a mini Neptune (Proxima c) orbits its parent star at a distance of 1.5 AU (224.4 million km; 139.4 million mi) and orbits it at a distance of 1.5 AU (224 .4 million km; 139.4 million). mi).
As of June 2020, a team of astronomers from the University of Texas McDonald Observatory had confirmed the presence of Proxima c from Hubble (25-year-old) radial velocity measurements. Their research further tightened the constraints on the planet’s mass and orbital period, which are now thought to be 0.8 Jupiter masses and 1900 days, respectively, based on their findings.
Astronomers at Australia’s Parkes Radio Telescope revealed in December 2020 that they had spotted an “enticing” radio signal coming from the direction of Proxima Centauri. The signal was discovered during a Breakthrough Listen to monitoring mission in April and May 2019. Breakthrough Listen Candidate 1 (BLC1) was a 30-hour signal with many unusual properties.
For example, the signal was a razor-sharp, narrow-band emission — at 982 megahertz (MHz) — that appeared to be shifting in frequency (also known as a Doppler shift). According to several astrophysicists, this is consistent with a shifting source (i.e. a planet orbiting it is the star). The scientific community, on the other hand, has now determined that the signal is most likely the result of natural processes.
Missions in progress
NASA launched the Transiting Exoplanet Survey Satellite (TESS) on April 18, 2018. This mission effectively followed in Kepler’s footsteps, monitoring thousands of stars simultaneously using the same procedure but superior instrumentation. TESS is now conducting the first space-based all-sky transit exoplanet survey using four wide-angle telescopes and associated charge-coupled device (CCD) detectors.
The main mission of TESS lasted two years and ended on July 5, 2020. On August 12, NASA announced a 27-month extension of the mission. TESS will re-observe the southern ecliptic hemisphere (which it observed during its main mission) during the first year of its expanded mission, and then spend the next 15 months observing the northern ecliptic hemisphere and 60 percent of the ecliptic.
TESS has scanned almost 75% of the sky during its main mission, examining 200,000 of the brightest stars near the Sun for signs of passing exoplanets. TESS has discovered 2,487 exoplanets and confirmed 107 as of February 6, 2021, ranging from terrestrial candidates to super Jupiters.
In addition, the European Space Agency’s (ESA) Gaia Observatory continued to track the precise locations, proper motions and orbits of over a billion stars, planets, comets, asteroids and quasars. This project became operational in 2013 (the same year that the European Space Agency’s Herschel space telescope was decommissioned) with a five-year main mission.
Gaia is now on a mission extension that will last until December 31, 2022, although it is expected to be extended again until December 31, 2025. The mission has been operational for seven years, one month and 18 days and will continue to survey the cosmos to create the world’s largest and most accurate 3D space database.
The ExOPlanets characterizing satellite (CHEOPS), launched on 18 December 2019 and the first small-class mission in ESA’s Cosmic Vision scientific programme, is another exoplanet-hunting mission controlled by ESA. Until the end of its main mission (scheduled for mid-2023), CHEOPS will study known exoplanets to obtain more accurate estimates of their mass, density, composition and formation.
There’s also the venerable Hubble Space Telescope, which has been in operation for more than 30 years! Hubble was instrumental in the discovery and characterization of exoplanets, in addition to making important discoveries that have transformed our understanding of the universe around us (e.g., calculating the rate of cosmic expansion that led to the notion of dark energy).
For example, early on in his mission, Hubble discovered debris disks around distant stars (from which planets form) and planetary systems in the process of formation. In the meantime, astronomers have been able to tap into Hubble’s archives and find evidence of planets passing in front of their stars, as well as spectra that have allowed exoplanet atmospheres to be characterized.
Hubble’s long-term observations helped astronomers do that, tooi to learn about the diversity of exoplanets and to develop the current classification system. Additionally, Hubble has taught astronomers much about the diversity of host stars and how their properties can affect a planet’s habitability.
Missions in the future
Several next-generation space telescopes will be launched in the coming years to aid in the ongoing search for habitable exoplanets. NASA’s long-awaited James Webb Space Telescope (JWST) will be deployed to its position at the Sun-Earth L2 Lagrange point on October 31, 2021. This project will be the largest and most advanced space telescope ever built, and once in place it will have to go through a lengthy deployment process.
The JWST will be able to detect lower mass exoplanets orbiting closer to their stars using its advanced infrared (IR) suite and light-blocking coronagraphs. Most rocky, Earth-like planets orbiting within a star’s habitable zone (and therefore considered “potentially habitable”) are expected to be discovered here.
Existing space telescopes lack the resolution and sensitivity needed to study these planets with direct imaging. Smaller, rocky planets have also proved unable to collect spectra when passing in front of their stars with existing telescopes. However, by assessing which IR wavelengths are absorbed and/or emitted, the JWST instruments will be able to assess the chemical composition of exoplanet atmospheres.
The Roman Space Telescope Nancy Grace, also called “Mother of Hubble”, is a follow-up mission. The Roman Space Telescope will be able to provide a region of the sky 100 times the size of Hubble’s image clarity by using a 2.4-meter (ft) primary mirror with the Wide-Field Instrument’s IR camera, a coronagraph, a spectrometer, and a huge field of view.
The European Space Agency is also working on a number of next-generation observatories, such as the PLAnetary Transits and Oscillations of Stars (PLATO) space telescope. This mission will look for planetary transits between up to a million stars, attempt to characterize their atmospheres, and characterize stars by measuring their vibrations. This will be the third mid-range mission in the European Space Agency’s Cosmic Vision program and will launch in 2022.
Cosmic Vision’s fourth medium mission, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, will follow (ARIEL). Scheduled for launch in 2029, this mission will study and characterize the composition and thermal structures of at least 1,000 known exoplanets as they pass in front of their stars.