Tau Bootis b was one of the first exoplanets discovered, just 16 years ago in 1996, and remains as one of the nearest exoplanets to us. But until now we’ve never actually seen it, only having knowledge of it because of the gravitational effects on its star.
The parent star of Tau Bootis b is itself easily seen with the naked eye, but the planet is not visible even though it’s a very large “hot Jupiter” that orbits very close to its parent star.
Similar to most planets, Tau Bootis b doesn’t transit the disc of its star (as Venus did during its recent transit). Until now, a planet transiting in front of its star was the only way to study its atmosphere. The properties of an exoplanet’s atmosphere are imprinted onto the star’s light as it passes through the planet’s atmosphere on its way to us. Since we’ve never witnessed Tau Bootis b transiting its star, we had, until now, never been able to study its atmosphere.
Now, after 15 years of effort trying to study the planet, astronomers have found a way to reliably probe the structure of its atmosphere, and accurately measure its mass. The researchers used the Cryogenic InfraRed Echelle Spectrometer (CRIRES) instrument on the Very Large Telescope (VLT) at ESO’s Paranal Observatory in Chile, using a combination of high-quality infrared observations, at wavelengths around 2.3 microns, with a new trick that teases out the weak signal of the planet from the much stronger one of the parent star. “At infrared wavelengths, the parent star emits less light than in the optical regime, so this is a wavelength regime favorable for separating out the dim planet’s signal.”
Lead author of the study Matteo Brogi (Leiden Observatory, the Netherlands) says: “Thanks to the high quality observations provided by the VLT and CRIRES we were able to study the spectrum of the system in much more detail than has been possible before. Only about 0.01% of the light we see comes from the planet, and the rest from the star, so this was not easy.”
Most planets around other stars were discovered because of their gravitational effects on their parent stars and not from direct observation, greatly limiting the information that can be gained about their mass. This method of discovery only allows a lower limit to be calculated for a planet’s mass.
The new technique pioneered by the researchers is much more comprehensive. Seeing the planet’s light directly allows them to measure the angle which the planet orbits at, and subsequently figure its mass accurately. “By tracing the changes in the planet’s motion as it orbits its star, the team has determined reliably for the first time that Tau Bootis b orbits its host star at an angle of 44 degrees and has a mass six times that of the planet Jupiter in our own Solar System.”
“The new VLT observations solve the 15-year old problem of the mass of Tau Bootis b. And the new technique also means that we can now study the atmospheres of exoplanets that don’t transit their stars, as well as measuring their masses accurately, which was impossible before,” says Ignas Snellen (Leiden Observatory, the Netherlands), co-author of the paper. “This is a big step forward.”
In addition to “detecting the glow of the atmosphere and measuring Tau Bootis b’s mass, the team has probed its atmosphere and measured the amount of carbon monoxide present, as well as the temperature at different altitudes by means of a comparison between the observations and theoretical models. A surprising result from this work was that the new observations indicated an atmosphere with a temperature that falls higher up. This result is the exact opposite of the temperature inversion — an increase in temperature with height — found for other hot Jupiter exoplanets.”
“The VLT observations show that high resolution spectroscopy from ground-based telescopes is a valuable tool for a detailed analysis of non-transiting exoplanets’ atmospheres. The detection of different molecules in future will allow astronomers to learn more about the planet’s atmospheric conditions. By making measurements along the planet’s orbit, astronomers may even be able to track atmospheric changes between the planet’s morning and evening.”
“This study shows the enormous potential of current and future ground-based telescopes, such as the E-ELT. Maybe one day we may even find evidence for biological activity on Earth-like planets in this way,” concludes Ignas Snellen.
Image Credits: ESO/Digitized Sky Survey 2; ESO, IAU and Sky & Telescope; ESO/L. Calçada