High-precision infrared velocimetry

The velocimetry method, also called the radial velocity method, has enabled the discovery of hundreds of exoplanets to date. This is notably the method that led to the discovery of the first planet around a Sun-like star, 51 Peg b.

This technique is based on the fact that a planet induces a small motion to the star it orbits. We often say that a planet is in orbit around a star but, to be truly accurate, the two bodies, the star and the planet, orbit around their common center of mass. This is what we can see in the video below. Planet is much less massive than its parent star, so the center of mass is much closer to the center of the star. This causes the velocity of the star to vary by a few km/h on a period that corresponds to the time it takes for the planet to do a complete orbit, typically ranging from days to years. It is possible to measure the periodic velocity change using very accurate spectrographs. When the star is approaching us or moving away from us, this lead to a slight blue or red shift in its spectral lines, a phenomenon called “Doppler effect“.

Illustration of the radial velocimetry method. A planet that orbits a star induces a movement that can be measured using high precision spectrographs. Credit: ESO/L. Calçada

The more massive and the closer the planet to its star, the greater the effect is, and the easier it is to detect is using this method. This explains why hot Jupiters, planets that are very massive and very close to their stars, were the first detected with this method. One advantage of this method is that it constrains the mass of the planet. This information can be combined with the planet’s size (which can be obtained if the planet transits in front of its star) to deduce its density and constrain its interior composition.

Currently, the best spectrographs (like HARPS, on the 3.6m of ESO) have a precision of approximately of 1 m/s (3.6 km/h). This precision is good enough to detect exoplanets that have masses and distance to their stars that are similar to that of Jupiter. However, it is not sufficient to detect planets that have masses and distances similar to the Earth, as the motion Earth induces on our Sun is about 10 cm/s.

Interest of infrared spectrographs for low-mass stars

Astronomers are particularly interested by planets which, like the Earth, are at just the right distance from their star so that water can exist in liquid form at their surface. The current generation of instruments don’t have a sufficient precision to detect terrestrial planets in this “habitable zone” around Sun-like stars. It is however much easier to detect planets in this zone around low-mass stars for many reasons:

  • These stars are less luminous, the habitable zone is much closer to the star, which means the effect the planet has on its star is greater;
  • The orbital periods of habitable-zone planets are much smaller for these stars, which simplifies follow-up;
  • The stars themselves being less massive, the center of mass of the system is farther from the star, which mean the effect is greater;
  • The low-mass stars are much more numerous than Sun-like stars. There are many in the Solar neighborhood, which multiplies the number of observable stars;
  • The first detections of planets around low-mass stars suggest that the majority of these stars have small, potentially rocky planets.

Even with these advantages, the detection of terrestrial planets in the habitable zone of low-mass stars however presents a significant challenge. These stars emit most of their light in the infrared portion of the spectrum, but current high precision spectrographs observe visible light.

The researchers at iREx are thus involved in the construction of two high-precision spectrographs that will operate in the infrared and allow the detection and characterization of terrestrial planets in the habitable zone of low-mass stars. The first, SPIRou, will be installed on Canada-France-Hawaii Telescope in 2018, while the second, NIRPS, will be installed in 2019 on the 3.6m telescope of ESO, in Chile.

To learn more about SPIRou and NIRPS, see