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SPHERE performance permits many kinds of challenges, from measuring astrometric microlensing to image very faint planets

August 14, 2018

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Alice Zurlo from the Astronomy Nucleus of UDP shows how with SPHERE we can learn more about exoplanets.

on-sky movement of Proxima Centauri with respect to background stars

The gravitational mass of Proxima Centauri measured with SPHERE from a microlensing event

Proxima Centauri, our closest stellar neighbor, is the subject of many attentions since the discovery of a telluric planet in 2016 by the technique of radial velocity. This planet would have a mass of 1.27 Earth masses and orbits the star in 11.2 days. Since the central star is a cool M dwarf the planet resides in the zone where water, if present at the surface of a rocky planet, can be potentially liquid, that is the so called habitable zone. However, the mass of the planet is not completely determined, because radial velocity measures in fact a mass ratio between the star and the planet. Therefore, the determination of the star’s mass is of the outmost importance to reveal the actual nature of the planet. So far this mass (0.12 solar mass) was inferred indirectly using a model linking the luminosity of a star to its mass, but this can be subject to large uncertainties especially at young ages and low masses. However, the gravitational mass of the star can be determined using the micro-lensing effect which causes the light of background stars to be deflected by a foreground object. Because Proxima Centauri is very close to the Solar System it moves significantly with respect to the background stars increasing the chance of line of sight alignements with other stars. Such an event occurred in 2016, and a team led by Alice Zurlo have used SPHERE to monitor precisely the astrometric position of one particular source at 9 epochs between 2015 and 2017. The figure shows the motion of Proxima as measured by SPHERE with respect to background stars. With this experiment the team was able to set a gravitational mass of 0.15 solar mass (with an accuracy of about 0.06 solar mass) shifting the mass of the planet to a slightly more massive value: 1.56 Earth masses. In this range of mass we can still expect the planet to be rocky. Another micro-lensing event caused by Proxima Cen happened last month and has been observed with SPHERE, which will allow us to refine even more the mass measurement.

Paper published in Monthly Notices of the Astronomical Royal Society, Zurlo et al, 2018

Imaging radial velocity planets with SPHERE

Radial Velocity (RV) is one of the most productive technique to detect planets orbiting around other stars. A lot has been learned from RV surveys on the ground about exoplanets demography. However, the mass of the planet that is inferred from this technique is a lower limit. Because the inclination of the orbit is unknown and since RV measures the velocity along the line of sight, in some cases these planets could be larger and massier. Of interest is the possibility to obtain an upper limit of this mass by other means and other techniques, like for instance direct imaging. But planets detected by RV are usually old and cool and may not emit enough light to be directly observed… unless they are bigger than expected (meaning their orbital plane is far from edge one).
The team led by Alice Zurlo have selected 5 stars with known planets conveniently located (far enough from the host star) and massive enough too (5 to 10 masses of Jupiter) for expecting a detection with SPHERE. None of these planets were detected although the contrast in the images reached very large values. These stars turned out to be too old (>1 billion years), such that the achieved contrasts are corresponding in fact to masses of 25 to 28 Jupiter masses. These estimates allow to derive limits on the orbital plane inclinations of 10 to 20 degrees depending on systems. Overall, all these systems likely harbor actual planets below 13 Jupiter masses, the standard mass for an object to be a planet. The next generation of extremely large telescopes while increasing the angular resolution will definitely be more efficient for this type of study.

Paper published by the Monthly Notices of the Astornomical Royal Society, Zurlo et al, 2018