Researchers from the Institute of Space Sciences (ICE, CSIC) and from Institute of Space Studies of Catalonia (IEEC) have participated in an international study carried out by the CARMENES consortium, which has discovered two small, terrestrial planets around Teegarden’s Star. The planets have masses similar to Earth and their temperatures could be mild enough to sustain liquid water on their surfaces. Observations were carried out with the CARMENES instrument in Calar Alto (Spain), as well as several other smaller complementary facilities, including IEEC’s Telescopi Joan Oró, at the Montsec Astronomical Observatory. The scientific paper is led by researchers at the University of Göttingen and appears in Astronomy & Astrophysics.
At a distance of only 12.5 light years, Teegarden’s Star is the 24th nearest star system to ours, and one of the smallest red dwarf stars known. Despite its proximity and due to its faintness, Teegarden’s Star was only identified in 2003.
“We have been observing this star with the CARMENES instrument since the beginning of the survey three years ago to measure its motion very precisely” explains Dr. Mathias Zechmeister, a postdoctoral researcher at the University of Göttingen (Germany), and lead author of the publication.
The method used to detect the planets was the so-called Doppler technique. A planet orbiting a star causes a small back-and-forth reflex motion. This motion induces a very subtle Doppler effect on the star light, which is then measured down to a precision of 1 meter per second with CARMENES, the equivalent of walking speed, or 3.6 km/h. Small planets produce a small signal, but their signals are easier to detect on small red dwarfs like Teegarden’s Star rather than on a star like the Sun because the reflex motion is larger and it repeats more often.
“CARMENES is the first high-precision spectrometer in operation specifically designed to find planets using this ‘red dwarf advantage’,” adds Mathias Zechmeister. Teegarden’s temperature is only 2600ºC (compared to the 5500ºC of the Sun), it is 1500 times fainter and ten times less massive than our Sun. As a result, it radiates most of its energy in the red and infrared which made it an ideal target for the CARMENES survey.
The Doppler measurements of Teegarden’s Star showed the presence of at least two signals, now reported as the two new exoplanets, Teegarden’s Star b and c. Obtaining a robust detection with a new instrument required the collection of over 200 measurements. Based on the measured motion, the researchers can deduce that Teegarden’s Star b has a mass similar to that of the Earth, it orbits the star every 4.9 days at about 2,5% the Earth-Sun distance. Teegarden’s Star c is also similar to the Earth in terms of mass, completes its orbit in 11.4 days and is located at 4,5% the Earth-Sun distance. Since Teegarden’s Star radiates much less energy than our Sun, the temperatures on these planets should be mild and they could, in principle, hold liquid water on their surfaces, especially the outer one, Teegarden’s Star c. This kind of planets is the primary target for future searches for life beyond our Solar System.
A major milestone of the CARMENES project
As opposed to previous CARMENES discoveries that combined measurements from several instruments, such as Barnard’s star b, all high-precision Doppler measurements and follow-up observations used for this finding have been obtained by the CARMENES consortium. Several groups within the consortium used smaller telescopes to monitor changes in the brightness of the star to rule out alternative explanations such as star spots or other surface features. The follow-up activities included intensive photometric campaigns at the 1.23-m Calar Alto Telescope/CSIC, the Sierra Nevada Observatory/IAA-CSIC and the Telescopi Joan Oró-Montsec/IEEC, among others.
“This discovery is a great success for the CARMENES project, which was specifically designed to search for planets around the least massive stars“, says Dr. Ignasi Ribas, a researcher from IEEC at ICE/CSIC, and project scientist of CARMENES. The new planets are number ten and eleven in the CARMENES exoplanet discovery tally, and the search continues.
“Both planets may be part of a larger system,” says Prof. Stefan Dreizler from University of Goettingen and co-author of the study. “Very low-mass stars seem to have densely populated planetary systems“ More data may reveal an even richer system.
“The unique feature of our instrument, which allows it to observe simultaneously in the visible and near infrared, is fundamental to confirm the nature of the signals detected with both channels as due to the presence of planets in orbit, since in this case, the amplitude of the signal does not depend on the channel with which it is measured, contrary to what happens when the signal is due to the star’s intrinsic variability,” points out Dr. Pedro Amado from IAA/CSIC, and deputy principal investigator of CARMENES.
The CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) instrument is a high-resolution optical and near-infrared spectrograph built in collaboration by 11 Spanish and German research institutions, and it is operated by the Calar Alto observatory (Spain). CARMENES has been working non-stop since 2016.