What Is An Exoplanet?
An exoplanet, or extrasolar planet, is a planet outside the solar system, usually orbiting another star in our galaxy. Most orbit another star but some are free-floating exoplanets, called rogue planets which orbit the galactic centre and are untethered to any star.
Most of the exoplanets discovered so far are in a relatively small region of our galaxy, the Milky Way. The first exoplanets were discovered in the 1990s and since then thousands have been identified. It’s known to be very rare for astronomers to see an exoplanet through a telescope the way you might see Saturn through a telescope from Earth.
This is called ‘direct imaging’ and only a tiny amount of discoveries have been made this way. NASA’s Kepler Space Telescope has discovered that there are more planets than stars in the galaxy.
The closest exoplanet to Earth is called Proxima Centauri b, this exoplanet is roughly 4 light-years away. The number of confirmed planets is in the thousands and rising rapidly, and as technology evolves and more can be seen, this number is likely to skyrocket.
In this article, we are going to talk about what techniques scientists have come up with to discover exoplanets, as it’s not as straightforward as ‘direct imaging’ discovery, as they are so far away often they are too small to identify, so clever techniques have been adopted to make the discoveries.
How Are Exoplanets Discovered?
There are five commonly used methods scientists use today to discover exoplanets.
Radial Velocity (Doppler Spectroscopy)
This method relies on observing the spectra stars for signs of ‘wobble’, where the star appears to be moving towards and away from Earth. This movement is caused by the presence of planets that exert a gravitational influence on their respective sun.
To put it simply, this method isn’t to search for planets themselves, but to search for the appearance of ‘wobbling’ planets. To do this, a spectometer is used to measure how the star’s spectral lines are displaced due to the Doppler Effect.
This is measured by how the light from the star is moving either towards the red or the blue spectrum, known as ‘redshift’, or ‘blueshift’, which determines whether it’s moving away or towards.
Redshift is the star moving away, and Blueshift is the star moving towards Earth. Based on the star’s velocity, astronomers can assess the presence of a planet or a cluster of planets. Until 2012 this method was the most effective method to discover exoplanets.
This method measures the light curve of distant stars for periodic dips in brightness. These are the result of exoplanets passing in front of the star causing a momentary dip in brightness. This dip is measured and the size and mass of the exoplanet can be calculated.
The reason they know it’s an exoplanet is the periodic dip of light which is the exact same each time the exoplanet orbits. Transit Photometry is considered a very reliable method of exoplanet detection.
Of the 3,526 exoplanets that have been discovered to this day, the transit method has accounted for 2,771 of them. This is more than all the other methods combined. One of the biggest advantages of Transit Photometry is the way it provides accurate information on the size of the planets detected.
This method is commonly combined with the Radial Velocity method which then determines the mass and density of the planet. From all this information, astronomers can assess a planet’s physical structure and composition, determining whether it’s a rocky planet or a gas giant.
Transit Photometry can also assess a planet’s atmosphere. As light from the star passes through the planet’s atmosphere, the resulting spectra can be analyzed to determine what elements are present, therefore proving clues to the chemical composition of the exoplanet’s atmosphere.
Another advantage of this technique is its ability to assess the planet’s temperature and radiation based on secondary eclipses.
In this moment of the eclipse, astronomers measure the star’s photometric intensity and then subtract it from the measurements of the planet’s temperature; this can even determine the presence of cloud formations in the atmosphere.
So far only a handful of exoplanets have been discovered using direct imaging. This is a challenging method compared to indirect methods however it’s the most promising when it comes to the characteristics of exoplanets atmospheres.
So far 100 planets have been confirmed in 82 planetary systems using this method, and many more are expected to be found in the near future. It’s quite clear from the name but direct imaging consists of taking images of exoplanets directly.
This is possible by searching for the light reflected from a planet’s atmosphere at infrared wavelengths. This is because, at infrared wavelengths, a star is only likely to be about 1 million times brighter than a planet reflecting light, rather than a billion times which is usually the case at visual wavelengths.
A big advantage of direct imaging is it’s far less prone to false findings, whereas the Transit Method is prone to false positives up to 40% of the time.
Although opportunities for using this method are rare, wherever it’s possible, it can provide very useful information on planets that other methods cannot.
For example, by examining the spectra reflected from a planet’s atmosphere, astronomers can obtain vital information about its composition, this information is key to determining if it’s potentially habitable.
Direct imaging is known to work best for planets that have wide orbits and are particularly massive like gas giants. It’s also very useful for detecting planets that are positioned ‘face on’, meaning that they don’t transit in front of the star relative to the observer.
This method relies on the gravitational force of distant objects to bend and focus light coming from a star. As a planet passes in front of the star relative to the observer, the light dips measurably, this can then be used as information to determine the presence of a planet.
Essentially, Gravitational Microlensing is a scaled-down version of Gravitational Lensing, where an intervening object is used to focus light coming from a galaxy or other mass located beyond it.
It also encompasses a key part of the Transit Method, where stars are assessed for dips in brightness to determine a presence of an exoplanet. Einstein’s Theory Of General Relativity states that gravity causes the fabric of spacetime to bend.
This effect can cause light affected by an object’s gravity to appear distorted or bent. It can also act as a lens, causing light to become more focused and making distant objects appear brighter.
This effect only happens when the two stars are almost exactly aligned relative to the observer. These events are called ‘lensing events’, and they are brief but common, as Earth and the stars in our galaxy are always moving relative to each other.
The greatest advantage of Microlensing is it’s the only known method capable of discovering planets at a huge distance away from Earth. This technique is capable of finding the smallest of exoplanets.
Microlensing is also proven to be the only method capable of detecting low mass planets in wider orbits, where both the transit method and radial velocity are useless.
A disadvantage to microlensing is that the events required are unique and not repeatable, any planet detected using this method will not be observable again, also, the planets that are detected tend to be extremely far away, which makes follow up studies practically impossible.
The good thing about this is microlensing detections don’t usually require follow up studies since they have a very high signal-to-noise ratio. Microlensing is unable to identify accurate estimates of a planet’s orbital properties since the only orbital characteristic that can be directly determined with this method is the planet’s current semi-major axis.
Therefore, a planet with an eccentric orbit will only be detectable for a tiny portion of its orbit.
Astrometry is the oldest method used to search for extrasolar planets. As early as 1943, astronomer Kaj Strand, working at the Sproul Observatory at Swarthmore College announced that his astrometric measurements revealed the presence of a planet orbiting the star 61 Cygni.
The announcement was greeted with enthusiasm at the time, but the claim has remained unproven and astronomers today are highly doubtful of Strand’s results.
Strands announcement was followed decades later by two other contentious claims. In 1960 Sproul astronomer Sarah Lippincott published a paper claiming that the star Lalande 21185 was orbited by a planet roughly ten Jupiter masses, in 1963 the observatories director, Peter Van de Kamp announced the discovery of a planet orbiting Barnard’s Star.
All these claims based on decades of meticulous observation were eventually cast into serious doubt, which shows the immense difficulties confronting the astrometric hunt for planets.
Until very recent history, the level of precision needed to detect the slight shifts in a star’s position that indicates the sign of a planet was at the far outer edge of technological feasibility.
As of February 2020, there is only one confirmed planet on the NASA exoplanet archive which is listed as an astrometric discovery, DENIS-P L082303.1-491201 b, also known as VB 10b.
However, follow up radial velocity studies of VB 10b did not detect the signal that would be expected based on the astrometric data, so most researchers consider it a false positive.
One big advantage of Astrometry is it’s one of the most sensitive methods of detection of extrasolar planets. Unlike transit photometry, astrometry doesn’t depend on the distant planet being in near-perfect alignment with the observer.
Therefore astrometry can be applied to a far greater number of stars. Also, unlike the radial velocity method, astrometry provides an accurate estimate of a planet’s mass and not just the minimum figure.
What Telescopes Are Used To Discover Exoplanets?
Thousands of exoplanets have been discovered and confirmed orbiting other stars. The first evidence of exoplanets dates to 1917 when Van Maanen identified the first polluted white dwarf, however, the first confirmed detection of an exoplanet did not come until the 1990s.
The discovery of exoplanets grew exponentially in the following years with the launch of the Kepler Space Telescope.
The Kepler mission was designed to survey our region of the Milky Way galaxy to discover hundreds of Earth-size and smaller planets in the ‘habitable zone’, also called the “Godilocks zone,” the area around a star where rocky planets could have liquid water on the surface, and determine the fraction of stars that might have such planets around them.
After the second of Kepler’s four gyroscope-like wheels failed in 2013, Kepler finished its prime mission that November and began its extended mission, K2. The spacecraft was retired in 2018, but Kepler data is still being used to find exoplanets (more than 2,700 confirmed so far).
NASA’s Spitzer Space Telescope (2013-2020) was not originally designed to search for exoplanets, but its infrared instruments made it an excellent exoplanet explorer.
It was used in the notable discovery of the TRAPPIST-1 system. In 2018 the Transiting Exoplanet Survey Satellite (TESS) was launched as a successor to Kepler to discover exoplanets in orbit around the brightest dwarf stars, the most common star type in our galaxy.
Future space missions such as NASA’s James Webb Space Telescope and the Nancy Grace Roman Space Telescope hold huge hope for what we can learn from exoplanets.
Through spectroscopy, reading light signatures, astronomers hope to learn more about planet atmospheres and the conditions of the planets themselves.
The way in which discovering exoplanets has evolved hugely over the years. It has made the biggest leaps in recent history with huge technological advancements needed to properly observe star movements and light to make the discovery.
In history, there were many reported discoveries of exoplanets using old methods, which subsequently got overturned with the rise of newer technologies.
The different methods used now to discover exoplanets each have their strengths and weaknesses, and can also be combined to gain more information, depending on the placement of the star being studied.
The advances made in recent history leave astronomers incredibly excited for the future of discovery, in and beyond our solar system.
Space is a vast and complex thing we still know so little about, but these recent discoveries made with a combination of these 5 incredible theories have advanced astronomers’ findings tenfold and are predicted to increase exponentially with the continuation of technology developments.
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