Astrometry with long baseline optical interferometry (direct photons) lead to the mass measurement of an exoplanet, β Pictoris c from the motion of its bigger "sister" β Pictoris b!

This is so cool, mind blowing 🤯: Exoplanet science is maturing with disruptive novel instruments like GRAVITY and the great ExoGRAVITY effort and generous collaboration led by Sylvestre Lacour. 

Urbain Le Verrier predicted the existence of Neptune using mathematics. Here, we achieved the mass measurement of an exoplanet, β Pictoris c from the motion of its bigger "sister" β Pictoris b!
Optical/NIR interferometry is saved and has now a long future at ESO and beyond, on the ground and in space!
I am so happy to be an insignificant part of this. In 2001 I tested integrated optics components as part of my master's internship between IPAG (then LAOG) and CEA/LETI. Then, there were H-band optimized 3-beam recombiners aimed at the IOTA interferometer in Arizona. The ~same technology is used in GRAVITY to recombine 4 telescope beams by pairs.

​The mass of Beta Pictoris c from Beta Pictoris b orbital motion.
S. Lacour, J. J. Wang, L. Rodet, M. Nowak, J. Shangguan, H. Beust, A.-M. Lagrange, R. Abuter, A. Amorim, R. Asensio-Torres, M. Benisty, J.-P. Berger, S. Blunt, A. Boccaletti, A. Bohn, M.-L. Bolzer, M. Bonnefoy, H. Bonnet, G. Bourdarot, W. Brandner, F. Cantalloube, P. Caselli, B. Charnay, G. Chauvin, E. Choquet, V. Christiaens, Y. Clénet, V. Coudé du Foresto, A. Cridland, R. Dembet, J. Dexter, P. T. de Zeeuw, A. Drescher, G. Duvert, A. Eckart, F. Eisenhauer, F. Gao, P. Garcia, R. Garcia Lopez, E. Gendron, R. Genzel, S. Gillessen, J. H. Girard, X. Haubois, G. Heißel, Th. Henning, S. Hinkley, S. Hippler, M. Horrobin, M. Houllé, Z. Hubert, L. Jocou, J. Kammerer, M. Keppler, P. Kervella, L. Kreidberg, V. Lapeyrère, J.-B. Le Bouquin, P. Léna, D. Lutz, A.-L. Maire, et al. (38 additional authors not shown)
http://arxiv.org/abs/2109.10671
We aim to demonstrate that the presence and mass of an Exoplanet can now be effectively derived from the astrometry of another Exoplanet. We combined previous astrometry of ββ Pictoris b with a new set of observations from the GRAVITY interferometer. The orbital motion of ββ Pictoris b is fit using Markov chain Monte Carlo simulations in Jacobi coordinates. The inner planet, ββ Pictoris c, was also reobserved at a separation of 96\,mas, confirming the previous orbital estimations. From the astrometry of planet b only, we can (i) detect the presence of ββ Pictoris c and (ii) constrain its mass to 10.04+4.53−3.10MJup10.04−3.10+4.53MJup. If one adds the astrometry of ββ Pictoris c, the mass is narrowed down to 9.15+1.08−1.06MJup9.15−1.06+1.08MJup. The inclusion of radial velocity measurements does not affect the orbital parameters significantly, but it does slightly decrease the mass estimate to 8.89+0.75−0.75MJup8.89−0.75+0.75MJup. With a semimajor axis of 2.68±0.022.68±0.02\,au, a period of 1221±151221±15 days, and an eccentricity of 0.32±0.020.32±0.02, the orbital parameters of ββ Pictoris c are now constrained as precisely as those of ββ Pictoris b. The orbital configuration is compatible with a high-order mean-motion resonance (7:1). The impact of the resonance on the planets' dynamics would then be negligible with respect to the secular perturbations, which might have played an important role in the eccentricity excitation of the outer planet.