New way of probing exoplanet atmospheres: Tau Boötis b revealed

By tracing the changes in the planet’s motion as it orbits its star, scientists have determined reliably for the first time that Tau Boötis b orbits its host star at an angle of 44°, has a mass six times that of Jupiter, and temperatures that decrease with altitude.

By ESO, Garching, Germany — Published: June 27, 2012

Artist’s impression of the exoplanet Tau Boötis b. Credit: ESO

For the first time, a clever new technique has allowed astronomers to study the atmosphere of an exoplanet in detail — even though it does not pass in front of its parent star. An international team has used the European Southern Observatory’s (ESO) Very Large Telescope (VLT) to directly catch the faint glow from the planet Tau (τ) Boötis b. They have studied the planet’s atmosphere and measured its orbit and mass precisely for the first time — in the process solving a 15-year-old problem. Surprisingly, the team also found that the planet’s atmosphere seems to be cooler higher up, the opposite of what was expected.

Tau Boötis b was one of the first exoplanets to be discovered back in 1996, and it is still one of the closest exoplanets known. Although its parent star is easily visible with the naked eye, the planet itself certainly is not, and up to now, it could only be detected by its gravitational effects on the star. Tau Boötis b is a large “hot Jupiter” planet orbiting close to its parent star.

Like most exoplanets, this planet does not transit the disk of its star (like the recent transit of Venus). Up to now, such transits were essential to allow the study of hot Jupiter atmospheres — when a planet passes in front of its star, it imprints the properties of the atmosphere onto the starlight. As no starlight shines through Tau Boötis b’s atmosphere toward us, this means the planet’s atmosphere could not be studied before.

But now, after 15 years of attempting to study the faint glow that is emitted from hot Jupiter exoplanets, astronomers have finally succeeded in reliably probing the structure of the atmosphere of Tau Boötis b and deducing its mass accurately for the first time. The team used the CRIRES instrument on the VLT at ESO’s Paranal Observatory in Chile. They combined high-quality infrared observations — at wavelengths around 2.3 microns — with a clever new trick to tease out the weak signal of the planet from the much stronger one from the parent star.

“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,” said Matteo Brogi from the Leiden Observatory in the Netherlands. “Only about 0.01 percent of the light we see comes from the planet, and the rest from the star, so this was not easy.”

The majority of planets around other stars were discovered by their gravitational effects on their parent stars, which limits the information that can be gleaned about their mass — they only allow a lower limit to be calculated for a planet’s mass. The new technique pioneered here is much more powerful. Seeing the planet’s light directly has allowed the astronomers to measure the angle of the planet’s orbit and work out its mass precisely. By tracing the changes in the planet’s motion as it orbits its star, the scientists have determined reliably for the first time that Tau Boötis b orbits its host star at an angle of 44° and has a mass six times that of Jupiter.

“The new VLT observations solve the 15-year-old problem of the mass of Tau Boötis b,” said Ignas Snellen from the Leiden Observatory in the Netherlands. “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. This is a big step forward.”

Besides detecting the glow of the atmosphere and measuring Tau Boötis b’s mass, the scientists have 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 the 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,” said Snellen. “Maybe one day we may even find evidence for biological activity on Earth-like planets in this way.”

User Comments

Be the first to leave your comment below!

Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Register Today!

RICHARD MCCONNELL said:

The article says that the orbit of this planet is angled at 44 degrees, but does not give a baseline: presumably the line of sight from Earth?
I agree with Mr Simerville that it is irritating to be told of a 'clever trick' and then not to be told what it was.

Submitted: 6/29/2012 1:31:27 PM (CST)

ETTORE GRECO from HAWAII said:

New and old science
Modern scientists have introduced Standard Model and String theory but still today these two remain inconclusive when attempting to explain how the Universe was created from those conditions. In their theories, these scientists are trying to explain the Creation with atomic particles already existing at the time of the Big Bang. The contradictions originate from Einstein's faulty conclusions and from the reverence shown by scientists afraid to take a new stand which is not aligned. Einstein believed that one fixed number of atoms had always existed and that atoms did not originate from waves (as it is in Wavevolution).
In some regards, the progress of science came to a halt since 1927 when the credibility of Quantum Mechanics and the concept of causality were placed in doubt. That year in Copenhagen, the scientist Heisenberg introduced the “uncertainty principle” and according to this it is not possible to exactly determine at the same Time the Space and Movement of a particle. The problem is that to obtain an exact position occupied in Space, for example, by an electron it takes the use of short wavelengths which transfer to the electron a great amount of energy turning that electron unpredictable and uncontrollable. Instead, to obtain a precise account of the Movement of the electron it takes a very small amount of light (very long wave) which results in a poor definition of the position in Space of that particle. The more exact and defined is the Space of a particle the more difficult is to know its Movement and vice versa. Heisenberg manifested his frustration in the inability to determine the exact position of an electron and with his uncertainty principle it was then decided that what could not be observed with Human eye will not be part of science (like also for Wavevolution).
But, the fact that we can not yet observe the exact Space and Movement of one electron should not imply that the electron does not occupy one position in Space. It is evident that our knowledge occurs through the perception of our senses but it should be equally evident that One World exists and It is independent from our senses.
The philosophy of Subjectivism from Bohr, Einstein and Heisenberg believes that when one electron or one photon is not observed it does not have any position and its position becomes materialized only as consequence of one Human observation. In the name of science, one arrogant assumption, to be able to judge what is Divine with one Human eye, was sealed by these three scientists since 1927, and made into a "law". Besides that, the theory of Relativity from Einstein reported that any Absolute could never exist, also eliminating the possibility of One Truth.
One new Progressive Science will have to become much more relevant before Humankind could see the first dawn of a certain Light.
www.wavevolution.org

Submitted: 6/27/2012 10:02:46 PM (CST)