An exoplanet is a “a planet that orbits a star outside the solar system.” Exoplanet detection can not only help spur imagination in the public to imagine what life would be like on other planets, but also help further planetary science by providing more data to analyze and refine theories. But as humanity lacks the capability to send spacecraft to these systems in a fast and timely manner, there needs to be ways to detect these exoplanets. If you’re curious about what the definition and classification of planets are, check out our article on the subject.
Using satellites and other existing methods, scientists have developed several options in their toolkit for exoplanet detection. Some methods are more successful than others and are preferable for scientific use as they are more reliable.
- Radial Velocity
When there is a star with a planet, the star will move in its own small orbit in response to the planet’s gravity, which leads to variations in the speed with which the star moves in regards to Earth. This movement and radial velocity can be deduced from the displacement in the parent star’s spectral lines due to the Doppler effect. Using this method, it is easier to detect planets around low-mass stars; however, it is also possible for this method to produce false signals in multi-planet and multi-star system. This method is also known as Doppler Spectroscopy.
- Transit Photometry
In this method of exoplanet discovery, scientists can detect these objects if a planet crosses or transits in front of its parent star’s disk. When this transit occurs, then the observed visual brightness of the star drops by a small amount, depending on the relative sizes of the star and the planet. Disadvantages of this method include planetary transits are observable only when the planet’s orbit happens to be perfectly aligned from the astronomers’ vantage point and a high rate of false detections. Therefore, to prevent false positives, additional methods are often required to confirm these discoveries.
- Reflection/Emission Modulations
This method uses reflected light variants from short-period planets that are in close orbits around their stars that go through phases from full to new and back again. This means that these phase transitions provide the planets with lots of starlight, that causes the planets to heat and make thermal emissions potentially detectable
- Relativistic Beaming
Detecting exoplanets from light variations uses the observed flux from the star due to its motion. First proposed in the early 2000s, this method is also known as Doppler beaming or Doppler boosting. As the planet tugs the star with its gravitation, the density of photons and therefore the apparent brightness of the star changes from observer’s viewpoint. This method can be used to determine the orbital eccentricity and the minimum mass of the planet. With this method, it is easier to detect massive planets close to their stars as these factors increase the star’s motion.
- Ellipsoidal Variations
Massive planets can cause slight tidal distortions to their host stars. When a star has a slightly ellipsoidal shape, its apparent brightness varies, depending if the oblate part of the star is facing the observer’s viewpoint. This method helps to determine the minimum mass of the planet, and its sensitivity depends on the planet’s orbital inclination.
- Pulsar timing
Although not designed to assist with discovering exoplanets, pulsar timing has been adapted to help exoplanet research. This method is based upon scientists finding and noticing slight anomalies in the timing of the observed radio pulses emitted by pulsar. Although pulsars are rare to find and it would be rare to find a large number of planets formed around pulsars, it can beneficial as it can help provide more information than other methods.
- Variable star timing
This method uses the photometrically from the Doppler shift of the pulsation frequency, without needing spectroscopy to detect exoplanets. This method is not as sensitive as the pulsar timing variation method, due to the periodic activity being longer and less regular. The ease of detecting planets around a variable star depends on the pulsation period of the star, the regularity of pulsations, the mass of the planet, and its distance from the host star.
- Transit Timing
When using this method, scientists examine whether transits occur with strict periodicity, or if there is a variation. When multiple transiting planets are detected, they can often be confirmed with the transit timing variation method. This is useful in planetary systems far from the Sun, where radial velocity methods cannot detect them due to the low signal-to-noise ratio.
- Transit duration variation
In this method, variations in duration of the transits can help astronomers detect circumbinary planets because the stars significantly alter the motion of the companion. This means that any transiting planet has significant variation in transit duration
- Eclipsing Binary Minima Timing
When a binary star system is aligned in a way that from the perspective of Earth, the stars pass in front of each other in their orbits, the system is called an “eclipsing binary” star system. The time of minimum light, when the star with the brighter surface is at least partially obscured by the disc of the other star, is called the primary eclipse, and approximately half an orbit later, the secondary eclipse occurs when the brighter surface area star obscures some portion of the other star. This method works by examining these minima in binary systems.
- Gravitational Microlensing
Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. If the foreground lensing star has a planet, then that planet’s own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing contributions at a reasonable rate. This method is most fruitful for planets between Earth and the center of the galaxy, as the galactic center provides a large number of background stars.
- Direct Imaging
Although this method only provides loose constraints on planetary mass, this is a potential alternative to direct viewing. By directly imaging planets that are detected using their thermal emissions, scientists are able to detect the thermal radiation being radiated from the parent star. This. Allows the planet to become brighter than it would be then at visible wavelengths.
Light reflected off of the planets atmosphere, the light waves interact with the molecules in the atmosphere and become polarized. A major advantage for this method is that it allows for determination of the composition of the planet’s atmosphere. The main drawback is that it will not be able to detect planets without atmospheres. Astronomical devices used for polarimetry, called polarimeters, are capable of detecting polarized light and rejecting unpolarized beams.
By precisely measuring a star’s position in the sky and observing how that position changes over time, scientists can determine the nature of the observed bodies. This method can be used to find exoplanets because scientists would notice that the star itself to move in a tiny circular or elliptical orbit. This makes it easier to find planets around low-mass stars, especially brown dwarfs, using this method.
There are a few other possible methods that scientists can use to detect exoplanets, none of which have net results yet. All of these ideas below have the potential to become viable options for detecting exoplanets, but might not currently be utilized in a widespread manner yet.
- Flare and Variability Echo Detection
This method of exoplanet detect utilizes the faint light curve produced by non-periodic variability events that reflect off an exoplanet. Recent advances has led scientists to reexamine this method of exoplanet detection as these types of exoplanet echoes are predicted to be recoverable from high-cadence photometric and spectroscopic measurements of active star systems. If that were to be the case, then these echoes are theoretically observable in all orbital inclinations.
- Transit Imaging
This method works by imaging a star’s surface during a transit event to allow scientists to see the shadow of the planet transiting. This method could provide a direct measurement of the planet’s angular radius and the actual radius.
- Magnetospheric Radio Emissions
Scientists could utilize radio telescopes to detect radio emissions from exoplanet magnetospheres, which could enable determine planetary information such as rotation rate. This is crucial as determining planetary rotation rate of exoplanets would otherwise be difficult to detect.
- Auroral Radio Emissions
Using radio telescopes such as LOFAR, scientists could detect auroral radio emissions from giant planets that have plasma sources. An example that we can relate to is the volcanic moon of Jupiter Io.
- Modified Interferometry
By looking at the results of an interferogram using a Fourier-Transform-Spectrometer, scientists can detect signals from Earth-like exoplanets.