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It is exciting to see that the nearest star to our Sun has more than one planet (Proxima b, [1]). In a paper led by Mario Damasso and Fabio Del Sordo [2] revealed the existence of another one. The Proxima c is a planet with an orbital period of about 1900 days (~5.21 years), a semi-major axis a_c = 1.48 ± 0.08 AU, minimum mass (m_c sin i_c) = 5.8 ± 1.9 M⊕, and equilibrium temperature Teq=39(+16/−18) K. This super-Earth is found further away than the typical distances for this type of planets, something that poses interesting questions with respect to the planet formation and evolution.

Apart from the very interesting results of this paper what is equally exciting is the fact the Fabio has been my office mate since 2017 at the University of Crete. I  am really happy for him and his success. And, as he moved to a new position in Italy, I wish him all the best and new discoveries.

Abstract
Our nearest neighbor, Proxima Centauri, hosts a temperate terrestrial planet. We detected in radial velocities evidence of a possible second planet with minimum mass m_c sin i_c = 5.8 ± 1.9M_⊕ and orbital period Pc=5.21+0.26−0.22 years. The analysis of photometric data and spectro-scopic activity diagnostics does not explain the signal in terms of a stellar activity cycle, but follow-up is required in the coming years for confirming its planetary origin. We show that the existence of the planet can be ascertained, and its true mass can be determined with high accuracy, by combining Gaia astrometry and radial velocities. Proxima c could become a prime target for follow-up and characterization with next-generation direct imaging instrumentation due to the large maximum angular separation of ~1 arc second from the parent star. The candidate planet represents a challenge for the models of super-Earth formation and evolution.

References:

[1] Anglada-Escudé et al. 2016, “A terrestrial planet candidate in a temperate orbit around Proxima Centauri“, Nature, 536, 437 | NASA/ADS

[2] Mario Damasso, Fabio Del Sordo, Guillem Anglada-Escudé, Paolo Giacobbe, Alessandro Sozzetti, Alessandro Morbidelli, Grzegorz Pojmanski, Domenico Barbato, R. Paul Butler, Hugh R. A. Jones, Franz-Josef Hambsch, James S. Jenkins, María José López-González, Nicolás Morales, Pablo A. Peña Rojas, Cristina Rodríguez-López, Eloy Rodríguez, Pedro J. Amado, Guillem Anglada, Fabo Feng and Jose F. Gómez, 2020, “A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU”, Science Advances, Vol. 6, no. 3, eaax7467 | NASA/ADS

In Cassan et al. 2012 (Nature, 481, 167), “One or more bound planets per Milky Way star from microlensing observations“) we read the following very interesting statement:

Here we report a statistical analysis of microlensing data (gathered in 2002–07) that reveals the fraction of bound planets 0.5–10 AU (Sun–Earth distance) from their stars. We find that 17 +6/-9 % of stars host Jupiter-mass planets (0.3–10 MJ , where MJ = 318 M⊕ and M⊕ is Earth’s mass). Cool Neptunes (10–30 M⊕) and super-Earths (5–10 M⊕ ) are even more common: their respective abundances per star are 52 +22/-29 % and 62 +35/-37 %. We conclude that stars are orbited by planets as a rule, rather than the exception.

Trawling for transits in a sea of noise: A Search for Exoplanets by Analysis of WASP Optical Lightcurves and Follow-up (SEAWOLF)

E. Gaidos, D. R. Anderson, S. Lepine, K. D. Colon, G. Maravelias, N. Narita, E. Chang, J. Beyer, A. Fukui, J. D. Armstrong, A. Zezas, B. J. Fulton, A. W. Mann, R. G. West, F. Faedi

Studies of transiting Neptune-size planets orbiting close to nearby bright stars can inform theories of planet formation because mass and radius and therefore mean density can be accurately estimated and compared with interior models. The distribution of such planets with stellar mass and orbital period relative to their Jovian-mass counterparts can test scenarios of orbital migration, and whether “hot” (period < 10d) Neptunes evolved from “hot” Jupiters as a result of mass loss. We searched 1763 late K and early M dwarf stars for transiting Neptunes by analyzing photometry from the Wide Angle Search for Planets and obtaining high-precision (<10−3) follow-up photometry of stars with candidate transit signals. One star in our sample (GJ 436) hosts a previously reported hot Neptune. We identified 92 candidate signals among 80 other stars and carried out 148 observations of predicted candidate transits with 1-2 m telescopes. Data on 70 WASP signals rules out transits for 39 of them; 28 other signals are ambiguous and/or require more data. Three systems have transit-like events in follow-up photometry and we plan additional follow-up observations. On the basis of no confirmed detections in our survey, we place an upper limit of 10.25% on the occurrence of hot Neptunes around late K and early M dwarfs (95% confidence). A single confirmed detection would translate to an occurrence of 5.3±4.4%. The latter figure is similar to that from Doppler surveys, suggesting that GJ 436b may be the only transiting hot Neptune in our sample. Our analysis of Kepler data for similar but more distant late-type dwarfs yields an occurrence of 0.32±0.21%. Depending on which occurrence is applicable, we estimate that the Next Generation Transit Survey will discover either ~60 or ~1000 hot Neptunes around late K and early M-type dwarfs.

arXiv:1310.7586

It is always fascinating when science fiction becomes reality. This is the case with the Kepler-16b, a binary system that proved to host a cold giant planet (like Saturn), which compiles from gas and rocks. This opens a wide new category of planetary systems since most of the stars belong to a binary system.

For more information see NASA’s Kepler announcement and the related page with star/planet’s properties, along with the publication (Doyle L et al 2011: Science – in press, arXiv-1109.3432).

An amazing picture was published at the APOD today ! It shows all 1235 candidate planets discovered by Kepler in the same relative scale along with the Sun (top right … the lonely star) with the silhouettes of Earth and Jupiter. Somebody has to search deep in the high resolution image to spot the planets (all of them in … transit !).

Kepler’s candidate transiting planets
Credit to Jason Rowe & Kepler Mission

Kepler is a space based telescope to study a small area between Cygnus and Lyra for exoplanet transits of all possible targets but mainly to find Earth sized candidates in the habitable zone. A fist publication reveals these numbers:

“On 1 February 2011 the Kepler Mission released data for 156,453 stars observed from the beginning of the science observations on 2 May through 16 September 2009. There are 1235 planetary candidates with transit like signatures detected in this period. These are associated with 997 host stars. Distributions of the characteristics of the planetary candidates are separated into five class-sizes; 68 candidates of approximately Earth-size (radius < 1.25 Earth radii), 288 super-Earth size (1.25 Earth radii < radius < 2 Earth radii), 662 Neptune-size (2 Earth radii < radius < 6 Earth radii), 165 Jupiter-size (6 Earth radii < radius < 15 Earth radii), and 19 up to twice the size of Jupiter (15 Earth radii < radius < 22 Earth radii). In the temperature range appropriate for the habitable zone, 54 candidates are found with sizes ranging from Earth-size to larger than that of Jupiter. Five are less than twice the size of the Earth. Over 74% of the planetary candidates are smaller than Neptune. The observed number versus size distribution of planetary candidates increases to a peak at two to three times Earth-size and then declines inversely proportional to area of the candidate. Our current best estimates of the intrinsic frequencies of planetary candidates, after correcting for geometric and sensitivity biases, are 6% for Earth-size candidates, 7% for super-Earth size candidates, 17% for Neptune-size candidates, and 4% for Jupiter-size candidates. Multi-candidate, transiting systems are frequent; 17% of the host stars have multi-candidate systems, and 33.9% of all the candidates are part of multi-candidate systems.” [ Borucki WJ et al 2011, arXiv1102.0541v1 ]

And all these in 105 square degrees field of view which is (well, rather large for an astronomical instrument) only a small part of the whole sky, only 1/400 ! The numbers are more exciting if we include the fact that all of these systems are transits which means that a specific geometry is needed in order to see the dimming of the stars’ brightness due to the pass of the planet(s) in front of them. But more time is needed to validate these results as the transits must been seen again and again in order to be sure that what is seen is real and not something else, that’s why they are candidates and not confirmed planets yet. But still the numbers are huge and … promising!

More exciting results include the fact that before Kepler none Earth-sized candidates or candidates inside the habitable zone were known and now there are 68 and 54 candidates respectively. In addition, complex systems are discovered like the Kepler-11 which has 6 planets [ Lissauer JL et al 2011, Nature &  arXiv1102.0291 ] and a rocky planet [Batalha NM et 2011 – arXiv1102.0605 ].

As stated by the NASA Administrator Charles Bolden: ” In one generation we have gone from extraterrestrial planets being a mainstay of science fiction, to the present, where Kepler has helped turn science fiction into today’s reality “.