What is the Radial Velocity Method?

Welcome back to our series on Exoplanet-Hunting methods! Today, we look at another widely-used and popular method of exoplanet detection, known as the Radial Velocity (aka. Doppler Spectroscopy) Method.

The hunt for extra-solar planets sure has heated up in the past decade or so! Thanks to improvements made in instrumentation and methodology, the number of exoplanets discovered (as of December 1st, 2017) has reached 3,710 planets in 2,780 star systems, with 621 system boasting multiple planets. Unfortunately, due to the limits astronomers are forced to contend with, the vast majority have been discovered using indirect methods.

When it comes to these indirect methods, one of the most popular and effective is the Radial Velocity Method – also known as Doppler Spectroscopy. This method relies on observing the spectra stars for signs of “wobble”, where the star is found to be moving towards and away from Earth. This movement is caused by the presence of planets, which exert a gravitational influence on their respective sun.

Description:

Essentially, the Radial Velocity Method consists not of looking for signs of planets themselves, but in observing a star for signs of movement. This is deduced by using a spectometer to measure the way in which the star’s spectral lines are displaced due to the Doppler Effect – i.e. how light from the star is shifted towards the red or blue end of the spectrum (redshift/blueshift).

Diagram detailing the Radial Velocity (aka. Doppler Shift) method. Credit: Las Cumbres Observatory

These shifts are indications that the star is moving away from (redshift) or towards (blueshift) Earth. Based on the star’s velocity, astronomers can determine the presence of a planet or system of planets. The speed at which a star moves around its center of mass, which is much smaller than that of a planet, is nevertheless measurable using today’s spectrometers.

Until 2012, this method was the most effective means of detecting exoplanets, but has since come to be replaced by the Transit Photometry. Nevertheless, it remains a highly effective method and is often relied upon in conjunction with the Transit Method to confirm the existence of exoplanets and place constraints on their size and mass.

Advantages:

The Radial Velocity method was the first successful means of exoplanet detection, and has had a high success rate for identifying exoplanets in both nearby (Proxima b and TRAPPIST-1‘s seven planets) and distant star systems (COROT-7c). One of the main advantages is that it allows for the eccentricity of the planet’s orbit to be measured directly.

The radial velocity signal is distance-independent, but requires a high signal-to-noise-ratio spectra to achieve a high degree of precision. As such, it is generally used to look for low-mass planets around stars that are within 160 light-years from Earth, but can still detect gas giants up to a few thousand light years away.

The radial velocity technique is able to detect planets around low-mass stars, such as M-type (red dwarf) stars. This is due to the fact that low mass stars are more affected by the gravitational tug of planets and because such stars generally rotate more slowly (leading to more clear spectral lines). This makes the Radial Velocity Method highly useful for two reasons.

For one, M-type stars are the most common in the Universe, accounting for 70% of stars in spiral galaxies and 90% of stars in elliptical galaxies. Second, recent studies have indicated that low-mass, M-type stars are the most likely place to find terrestrial (i.e. rocky) planets. The Radial Velocity Method is therefore well-suited for the study of Earth-like planets that orbit closely to red dwarf suns (within their respective habitable zones).

Another major advantage is the way the Radial Velocity Method is able to place accurate constraints on a planet’s mass. Although the radial velocity of a star can only yield estimates a planet’s minimum mass, distinguishing the planet’s own spectral lines from those of the the star can yield measurements of the planet’s radial velocity.

This allows astronomers to determine the inclination of the planet’s orbit, which enables the measurement of the planet’s actual mass. This technique also rules out false positives and provides data about the composition of the planet. The main issue is that such detection is possible only if the planet orbits around a relatively bright star and if the planet reflects or emits a lot of light.

Number of extrasolar planet discoveries per year through September 2014, with colors indicating method of detection – radial velocity (blue), transit (green), timing (yellow), direct imaging (red), microlensing (orange). Credit: Public domain

As of December 2017, 662 of all exoplanet discoveries (both candidates and those that have been confirmed) were detected using the Radial Velocity Method alone – almost 30% of the total.

Disadvantages:

That being said, the Radial Velocity Method also has some notable drawbacks. For starters, it is not possible to observe hundreds or even thousands of stars simultaneously with a single telescope – as is done with Transit Photometry. In addition, sometimes Doppler spectrography can produces false signals, especially in multi-planet and multi-star systems.

This is often due to the presence of magnetic fields and certain types of stellar activity, but can also arise from a lack of sufficient data since stars are not generally observed continuously. However, these limitations can be mitigated by pairing radial velocity measurements with another method, the most popular and effective of which is Transit Photometry.

While distinguishing between the spectral lines of a star and a planet can allow for better constraints to be placed on a planet’s mass, this is generally only possible if the planet orbits around a relatively bright star and the planet reflects or emits a lot of light.  In addition, planet’s that have highly inclined orbits (relative to the observer’s line of sight) produce smaller visible wobbles, and are therefore harder to detect.

In the end, the Radial Velocity Method is most effective when paired with Transit Photometry, specifically for the sake of confirming detections made with the latter method. When both methods are used in combination, the existence of a planet can not only be confirmed, but accurate estimates of its radius and true mass can be made.

Examples of Radial Velocity Surveys:

Observatories that use the Radial Velocity method include the European Southern Observatory’s (ESO) La Silla Observatory in Chile. This facility conducts exoplanet-hunting surveys using its 3.6 meter telescope, which is equipped with the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrometer. There’s also the telescopes at the Keck Observatory in Mauna Kei, Hawaii, which rely on the High Resolution Echelle Spectrometer (HIRES) spectrometer.

There’s also the Haute-Provence Observatory in Southern France, which used the ELODIE spectrograph to detect 51 Pegasi b – the first “Hot Jupiter” found to be orbiting a main sequence star – in 1995. In 2006, ELODIE was decommissioned and replaced by the SOPHIE spectrograph.

Exoplanet-hunting surveys that rely on the Radial Velocity Method are expected to benefit greatly form the deployment of the James Webb Space Telescope (JWST), which is scheduled for 2019. Once operational, this mission will obtain Doppler measurements of stars using its advanced suite of infrared instruments to determine the presence of exoplanet candidates. Some of these will then be confirmed using the Transiting Exoplanet Survey Satellite (TESS) – which will deploy in 2018.

Thanks to improvements in technology and methodology, exoplanet discovery has grown by leaps and bounds in recent years. With thousands of exoplanets confirmed, the focus has gradually shifted towards the characterizing of these planets to learn more about their atmospheres and conditions on their surface. In the coming decades, thanks in part to the deployment of new missions, some very profound discoveries are expected to be made!

Sources:

Super-Earth Planet Found in the Habitable Zone of a Nearby Star

M-type stars, also known as “red dwarfs”, have become a popular target for exoplanet hunters of late. This is understandable given the sheer number of terrestrial (i.e. rocky) planets that have been discovered orbiting around red dwarf stars in recent years. These discoveries include the closest exoplanet to our Solar System (Proxima b) and the seven planets discovered around TRAPPIST-1, three of which orbit within the star’s habitable zone.

The latest find comes from a team of international astronomers who discovered a planet around GJ 625, a red dwarf star located just 21 light years away from Earth. This terrestrial planet is roughly 2.82 times the mass of Earth (aka. a “super-Earth”) and orbits within the star’s habitable zone. Once again, news of this discovery is prompting questions about whether or not this world could indeed be habitable (and also inhabited).

The international team was led by Alejandro Mascareño of the Canary Islands Institute of Astrophysics (IAC), and includes members from the University of La Laguna and the University of Geneva. Their research was also supported by the Spanish National Research Council (CSIS), the Institute of Space Studies of Catalonia (IEEC), and the National Institute For Astrophysics (INAF).

Diagram showing GJ 625’s habitable zone in comparison’s to the Sun’s. Credit: IAC

The study which details their findings was recently accepted for publication by the journal Astronomy & Astrophysics, and appears online under the title “A super-Earth on the Inner Edge of the Habitable Zone of the Nearby M-dwarf GJ 625“. According to the study, the team used radial-velocity measurements of GJ 625 in order to determine the presence of a planet that has between two and three times the mass of Earth.

This discovery was part of the HArps-n red Dwarf Exoplanet Survey (HADES), which studies red dwarf stars to determine the presence of potentially habitable planets orbiting them. This survey relies on the High Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) instrument – which is part of the 3.6-meter Galileo National Telescope (TNG) at the IAC’s Roque de Los Muchachos Observatory on the island of La Palma.

Using this instrument, the team collected high-resolution spectroscopic data of the GJ 625 system over the course of three years. Specifically, they measured small variations in the stars radial velocity, which are attributed to the gravitational pull of a planet. From a total of 151 spectra obtained, they were able to determine that the planet (GJ 625 b) was likely terrestrial and had a minimum mass of 2.82 ± 0.51 Earth masses.

Moreover, they obtained distance estimates that placed it roughly 0.078 AU from its star, and an orbital period estimate of 14.628 ± 0.013 days. At this distance, the planet’s orbit places it just within GJ 625’s habitable zone. Of course, this does not mean conclusively that the planet has conditions conducive to life on its surface, but it is an encouraging indication.

Tjhe Observatorio del Roque de los Muchachos, located on the island of La Palma. Credit: IAC

As Alejandro Suárez Mascareño explained in an IAC press release:

“As GJ 625 is a relatively cool star the planet is situated at the edge of its habitability zone, in which liquid water can exist on its surface. In fact, depending on the cloud cover of its atmosphere and on its rotation, it could potentially be habitable”.

This is not the first time that the HADES project detected an exoplanet around a red dwarf star. In fact, back in 2016, a team of international researchers used this project to discover 2 super-Earths orbiting GJ 3998, a red dwarf located about 58 ± 2.28 light years from Earth. Beyond HADES, this discovery is yet another in a long line of rocky exoplanets that have been discovered in the habitable zone of a nearby red dwarf star.

Such findings are very encouraging since red dwarfs are the most common type of star in the known Universe- accounting for an estimated 70% of stars in our galaxy alone. Combined with the fact that they can exist for up to 10 trillion years, red dwarf systems are considered a prime candidate in the search for habitable exoplanets.

But as with all other planets discovered around red dwarf stars, there are unresolved questions about how the star’s variability and stability could affect the planet. For starters, red dwarf stars are known to vary in brightness and periodically release gigantic flares. In addition, any planet close enough to be within the star’s habitable zone would likely be tidally-locked with it, meaning that one side would be exposed to a considerable amount of radiation.

Artist’s impression of of the exoplanets orbiting a red dwarf star. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).

As such, additional observations will need to be made of this exoplanet candidate using the time-tested transit method. According to Jonay Hernández – a professor from the University of La Laguna, a researcher with the IAC and one of the co-authors on the study – future studies using this method will not only be able to confirm the planet’s existence and characterize it, but also determine if there are any other planets in the system.

“In the future, new observing campaigns of photometric observations will be essential to try to detect the transit of this planet across its star, given its proximity to the Sun,” he said. “There is a possibility that there are more rocky planets around GJ 625 in orbits which are nearer to, or further away from the star, and within the habitability zone, which we will keep on combing”.

According to Rafael Rebolo – one of the study’s co-authors from the Univeristy of La Laguna, a research with the IAC, and a member of the CSIS – future surveys using the transit method will also allow astronomers to determine with a fair degree of certainty whether or not GJ 625 b has the all-important ingredient for habitability – i.e. an atmosphere:

“The detection of a transit will allow us to determine its radius and its density, and will allow us to characterize its atmosphere by the transmitted light observe using high resolution high stability spectrographs on the GTC or on telescopes of the next generation in the northern hemisphere, such as the Thirty Meter Telescope (TMT)”.

Artist’s impression of a system of exoplanets orbiting a low mass, red dwarf star. Credit: NASA/JPL

But what is perhaps most exciting about this latest find is how it adds to the population of extra-solar planets within our cosmic neighborhood. Given their proximity, each of these planets represent a major opportunity for research. And as Dr. Mascareño told Universe Today via email:

“While we have already found more than 3600 extra-solar planets, the exoplanet population in our near neighborhood is still somewhat unknown. At 21 ly from the Sun, GJ 625 is one of the 100 nearest  stars, and right now GJ 625 b is one of the 30 nearest exoplanets detected and the 6th nearest potentially habitable exoplanet.”

Once again, ongoing surveys of nearby star systems is providing plenty of potential targets in the search for life beyond our Solar System. And with both ground-based and space-based next-generation telescopes joining the search, we can expect to find many, many more candidates in the coming years. In the meantime, be sure to check out this animation of GJ 625 b and its parent star:

Further Reading: arXiv, IAC

Who Else Is Looking For Cool Worlds Around Proxima Centauri?

Finding exoplanets is hard work. In addition to requiring seriously sophisticated instruments, it also takes teams of committed scientists; people willing to pour over volumes of data to find the evidence of distant worlds. Professor Kipping, an astronomer based at the Harvard-Smithsonian Center for Astrophysics, is one such person.

Within the astronomical community, Kipping is best known for his work with exomoons. But his research also extends to the study and characterization of exoplanets, which he pursues with his colleagues at the Cool Worlds Laboratory at Columbia University. And what has interested him most in recent years is finding exoplanets around our Sun’s closest neighbor – Proxima Centauri.

Kipping describes himself as a “modeler”, combining novel theoretical modeling with modern statistical data analysis techniques applied to observations. He is also the Principal Investigator (PI) of The Hunt for Exomoons with Kepler (HEK) project and a fellow at the Harvard College Observatory. For the past few years, he and his team have been taking the hunt for exoplanets to the local stellar neighborhood.

The inspiration for this search goes back to 2012, when Kipping was at a conference and heard the news about a series of exoplanets being discovery around Kepler 42 (aka. KOI-961). Using data from the Kepler mission, a team from the California Institute of Technology discovered three exoplanets orbiting this red dwarf star, which is located about 126 light years from Earth.

At the time, Kipping recalled how the author of the study – Professor Philip Steven Muirhead, now an associate professor at the Institute for Astrophysical Research at Boston University – commented that this star system looked a lot like our nearest red dwarf stars – Barnard’s Star and Proxima Centauri.

In addition, Kepler 42’s planets were easy to spot, given that their proximity to the star meant that they completed an orbital period in about a day. Since they pass regularly in front of their star, the odds of catching sight of them using the Transit Method were good.

As Prof. Kipping told Universe Today via email, this was the “ah-ha moment” that would inspire him to look at Proxima Centauri to see if it too had a system of planets:

“We were inspired by the discovery of planets transiting KOI-961 by Phil Muirhead and his team using the Kepler data. The star is very similar to Proxima, a late M-dwarf harboring three sub-Earth sized planets very close to the star. It made me realize that if that system was around Proxima, the transit probability would be 10% and the star’s small size would lead to quite detectable signals.”

The MOST satellite, a Canadian built space telescope. Credit: Canadian Space Agency
The MOST satellite, a Canadian built space telescope. Credit: Canadian Space Agency

In essence, Kipping realized that if such a planetary system also existed around Proxima Centauri, a star with similar characteristics, then they would very easy to detect. After that, he and his team began attempting to book time with a space telescope. And by 2014-15, they had been given permission to use the Canadian Space Agency’s Microvariability and Oscillation of Stars (MOST) satellite.

Roughly the same size as a suitcase, the MOST satellite weighs only 54 kg and is equipped with an ultra-high definition telescope that measures just 15 cm in diameter. It is the first Canadian scientific satellite to be placed in orbit in 33 years, and was the first space telescope to be entirely designed and built in Canada.

Despite its size, MOST is ten times more sensitive than the Hubble Space Telescope. In addition, Kipping and his team knew that a mission to look for transiting exoplanets around Proxima Centauri would be too high-risk for something like Hubble. In fact, the CSA initially rejected their applications for this same reason.

“MOST initially denied us because they wanted to look at Alpha Centauri following the announcement by Dumusque et al. of a planet there,” said Kipping. “So understandably Proxima, for which no planets were known at the time, was not as high priority as Alpha Cen. We never even tried for Hubble time, it would be a huge ask to stare HST at a single star for months on end with just a a 10% chance for success.”

Artist's impression of the Earth-like exoplanet discovered orbiting Alpha Centauri B iby the European Southern Observatory on October 17, 2012. Credit: ESO
Artist’s impression of the Earth-like exoplanet discovered orbiting Alpha Centauri B iby the European Southern Observatory on October 17, 2012. Credit: ESO

By 2014 and 2015, they secured permission to use MOST and observed Proxima Centauri twice – in May of both years. From this, they acquired a month and half’s-worth of space-based photometry, which they are currently processing to look for transits. As Kipping explained, this was rather challenging, since Proxima Centauri is a very active star – subject to star flares.

“The star flares very frequently and prominently in our data,” he said. “Correcting for this effect has been one the major obstacles in our analysis. On the plus side, the rotational activity is fairly subdued. The other issue we have is that MOST orbits the Earth once every 100 minutes, so we get data gaps every time MOST goes behind the Earth.”

Their efforts to find exoplanets around Proxima Centauri are especially significant in light of the European Southern Observatory’s recent announcement about the discovery of a terrestrial exoplanet within Proxima Centauri’s habitable zone (Proxima b). But compared to the ESO’s Pale Red Dot project, Kipping and his team were relying on different methods.

As Kipping explained, this came down to the difference between the Transit Method and the Radial Velocity Method:

“Essentially, we seek planets which have the right alignment to transit (or eclipse) across the face of the star, whereas radial velocities look for the wobbling motion of a star in response to the gravitational influence of an orbiting planet. Transits are always less likely to succeed for a given star, because we require the alignment to be just right. However, the payoff is that we can learn way more about the planet, including things like it’s size, density, atmosphere and presence of moons and rings.”

Artist’s impression of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser
Artist’s impression of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser

In the coming months and years, Kipping and his team may be called upon to follow up on the success of the ESO’s discovery. Having detected Proxima b using the Radial Velocity method, it now lies to astronomers to confirm the existence of this planet using another detection method.

In addition, much can be learned about a planet through the Transit Method, which would be helpful considering all the things we still don’t know about Proxima b. This includes information about its atmosphere, which the Transit Method is often able to reveal through spectroscopic measurements.

Suffice it to say, Kipping and his colleagues are quite excited by the announcement of Proxima b. As he put it:

“This is perhaps the most important exoplanet discovery in the last decade. It would be bitterly disappointing if Proxima b does not transit though, a planet which is paradoxically so close yet so far in terms of our ability to learn more about it. For us, transits would not just be the icing on the cake, serving merely as a confirmation signal – rather, transits open the door to learning the intimate secrets of Proxima, changing Proxima b from a single, anonymous data point to a rich world where each month we would hear about new discoveries of her nature and character.”

This coming September, Kipping will be joining the faculty at Columbia University, where he will continue in his hunt for exoplanets. One can only hope that those he and his colleagues find are also within reach!

Further Reading: Cool Worlds