Have We Discovered The Smallest Extra-solar Planet?

Using pulsar timing techniques at the decimeter radio telescope of the Nançay Radio Observatory, an international team comprising researchers from CNRS, Observatoire de Paris, LPC2E in Orléans, the Max Planck Institute and the University of Aalborg in Denmark, has discovered a trace of what may well be the smallest extra-solar planet ever discovered in the system of pulsar PSR J0337+1715.

In 1992, the very first planetary system was discovered around the millisecond pulsar PSR B1257+12 [1] thanks to the measurement of arrival times [2]. In 2014, the pulsar PSR J0337+1715 was found at the heart of an exceptional three-star system [3] . A publication to be published in A&A today proposes the existence around this triple system of a planet less massive than the Moon or similar to the dwarf planet Pluto, making it potentially the smallest extra-solar planet ever discovered to date.

PSR J0337+1715 is exceptional: it is the only pulsar to be found in a triple star system where the three stars are located within a perimeter comparable to the size of the Earth’s orbit, i.e. 1 astronomical unit. This original configuration had already enabled unprecedented tests of general relativity [4] .

However, something wasn’t quite right: a residual signal, not taken into account by the mathematical models, remained. In trying to understand the nature of this signal, the researchers concluded that it could be a tiny planet orbiting at a distance from the triple system, with an orbital period of around 8 years.

An alternative hypothesis is that the pulsar itself produces a random signal, a phenomenon known to temporarily “mimic” the signal of a planet. Here, however, the signal amplitude is unusually large. The predictions obtained under the two hypotheses differ over time, so observations over the next few years will be crucial in deciding the question.

Reference :

Explanation of the exceptionally strong timing noise of PSR J0337+1715 by a circum-ternary planet and consequences for gravity tests, G. Voisin, I. Cognard, M. Saillenfest, T.M. Tauris, N. Wex, L. Guillemot, G. Theureau, P.C. Freire, M. Kramer. Astronomy and Astrophysics, Forthcoming article

[1] Wolszczan, A. & Frail, D. A. A planetary system around the millisecond pulsar PSR1257 + 12. Nature 355, 145-147 (1992)

[2] The chronometry technique is based on the extremely precise measurement, to the nearest microsecond, of the arrival times of pulses received from this type of neutron star known as a pulsar. This technique is extremely sensitive to the slightest gravitational disturbance. In fact, if the pulsar moves just 300m, the pulses arrive earlier or later, depending on whether the star has moved closer to or further away from the solar system. A very fine mathematical reconstruction makes it possible to link these disturbances to the presence of companions and to deduce their characteristics

[3] Ransom, S. M. et al. A millisecond pulsar in a stellar triple system. Nature 505, 520-524 (2014)

[4] Two independent studies :

Archibald, A. M. et al. Universality of free fall from the orbital motion of a pulsar in a stellar triple system. Nature 559, 73-76 (2018) Voisin, G. et al. An improved test of the strong equivalence principle with the pulsar in a triple star system. A&A 638, A24 (2020)

Astrobiology