Returning to our exoplanet hunting methods series! Today we look at the radial velocity method (also known as Doppler spectroscopy), another widely used and popular method for finding exoplanets.
The search for extrasolar planets has gotten a lot hotter in the last decade or two! The number of exoplanets discovered (as of December 1, 2017) has risen to 3,710 planets in 2,780 star systems with 621 plans enjoying multiple worlds, thanks to advances in instrumentation and the methods. Unfortunately, due to the limitations faced by astronomers, the vast majority have been identified through indirect methods.
When it comes to indirect approaches, one of the most popular and effective is the radial velocity method, commonly known as Doppler spectroscopy. This method focuses on looking for “wobble signals” in the spectra of stars, where the star appears to be moving toward or away from Earth. The presence of planets that exert a gravitational effect on their suns causes this movement.
The radial velocity method is essentially a method of examining a star for motion signals, rather than looking for signs of planets. This is determined by using a spectrometer to detect the shift in the star’s spectral lines due to the Doppler effect, i.e. H. how light is shifted to the red or blue end of the spectrum (redshift/blueshift).
The radial velocity approach (aka Doppler shift) is shown in this graph. Las Cumbres Observatory is the source of this image.
These shifts indicate whether the star is moving away from (redshift) or towards (blueshift) Earth. Astronomers can use the speed of the star to determine the presence of a planet or planetary system. Even though the speed at which a star revolves around its center of mass is much slower than that of a world, spectrometers can measure it.
This method was the most effective method for detecting exoplanets until 2012, when transit photometry surpassed it. Nonetheless, it is a highly effective method, often used in conjunction with the transit method, to confirm the presence of exoplanets and impose size and mass restrictions on them.
Advantages: The radial velocity approach was the first effective method for detecting exoplanets. and it has a high success rate in detecting exoplanets in both near (Proxima b and the seven planets of TRAPPIST -1) and distant star systems (Proxima b and the seven planets of TRAPPIST -1) (COROT-7c). One of its main advantages is that it allows a direct measurement of the eccentricity of the planet’s orbit.
The radial velocity signal is independent of distance, although it requires a high signal-to-noise spectrum to achieve good accuracy. As a result, it is most commonly used to find low-mass planets orbiting stars within 160 light-years of Earth, but it can also detect gas giants up to a few thousand light-years away.
Planets surrounding low-mass stars, such as B. M-type stars (red dwarfs) can be detected using the radial velocity approach. This is because the gravitational pull of planets acts on low-mass stars, and low-mass stars rotate more slowly (resulting in clearer spectral lines). The radial velocity method is very valuable for two reasons.
First, M-type stars are the most common in the universe, making up 70% of spiral galaxies and 90% of elliptical galaxies ” stars. Second, new research suggests that low-mass M-type stars are likely candidates for rocky planets. Therefore, the radial velocity method is well suited to study Earth-like planets orbiting red dwarf suns (within their respective habitable zones). .
Another significant advantage is the ability of the radial velocity method to impose specific constraints on a planet’s massshoot. Although the radial velocity of a star can only estimate the minimum mass of a planet, separating the planet’s spectral lines from those of the principal can provide measurements of the planet’s radial velocity.
Astronomers can use this information to calculate the inclination of the planet’s orbit, allowing them to calculate its actual mass. This method also eliminates false positives and provides information about the composition of the planet. The basic problem is that such an identification is only possible if the planet orbits a sufficiently very bright star and reflects or emits a lot of light.
By September 2014, the number of extrasolar planet discoveries has increased, with the colors denoting the detection technique – radial velocity (blue), transit (green), timing (yellow), direct imaging ( red) and microlenses (green) (orange). Public domain image
The radial velocity method was used to detect 662 of all exoplanet discoveries (including potential and confirmed) up to December 2017, accounting for about 30% of the total.
Disadvantages: However, the radial velocity method has several significant disadvantages. First of all, unlike transit photometry, it is not possible to observe hundreds or even thousands of stars simultaneously with a single telescope. In addition, Doppler spectrography can occasionally provide erroneous signals, especially in multi-planet and multi-star systems.
This is often related to the presence of magnetic fields and certain forms of stellar activity, but can also be due to a lack of data as stars are rarely studied continuously. By combining radial velocity measurements with another approach, the most popular and effective of which is transit photometry, these limitations can be overcome.
While distinguishing between a star’s and a planet’s spectral lines might help to more constrain a planet’s mass, this is usually only possible when the Earth orbits a relatively bright star and reflects or emits a lot of light. Planets with highly inclined orbits (relative to the observer’s line of sight) have smaller discernible variations, making them more difficult to identify.
Finally, the radial velocity method is most effective when used in conjunction with transit photometry, particularly for the purpose of validating detections obtained with the latter. When these methods are combined, it is possible to confirm the existence of a planet, as well as make accurate estimates of its radius and true mass.
Radial velocity measurements are used by observatories around the world, including the European Southern Observatory (ESO) La Silla Observatory in Chile. This facility uses its 3.6-meter telescope equipped with the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrometer to search for exoplanets. The High-Resolution Echelle Spectrometer (HIRES) spectrometer is also used by the telescopes at the Keck Observatory on Mauna Kei, Hawaii.
In 1995, the Haute-Provence Observatory in southern France used the ELODIE spectrograph to discover 51 Pegasi b and it was determined that the first “hot Jupiter” was a main sequence star circled. ELODIE was retired in 2006 and replaced by the SOPHIE spectrograph.
The expected 2019 deployment of the James Webb Space Telescope (JWST) is expected to give a significant boost to surveying exoplanets using the radial velocity method. Once operational, this mission will use its powerful infrared detectors to collect Doppler measurements of stars to assess the presence of exoplanet candidates. The Transiting Exoplanet Survey Satellite (TESS), which will be launched in 2018, will confirm some of these.
The search for exoplanets has come in leaps and bounds in recent years thanks to advances in technology and approach. With thousands of exoplanets verified, attention has increasingly turned to defining these planets to understand more about their atmospheres and surface properties. It is expected that in the following decades some outerSignificant discoveries are being made, partly thanks to the deployment of new missions!
Here at Universe Today we have a number of fascinating articles about exoplanets. What are extrasolar planets, what is the transit method, what is the direct imaging method, what is the gravitational microlensing method and Kepler’s universe: More planets in our galaxy than stars are among the topics covered in this article.
Astronomy Cast also has some episodes on this topic. HARPS Spectrograph is episode 366.
For more information, see NASA’s Exoplanet Exploration page, the Planetary Society’s extrasolar planets page, and the NASA/Caltech Exoplanet Archive.