At a distance of 1.295 parsecs, 1 the red-dwarf Proxima Centauri (α Centauri C, GL 551, HIP 70890, or simply Proxima) is the Sun's closest stellar neighbour and one of the best studied low-mass stars. It has an effective temperature of only ∼ 3050 K, a luminosity of ∼0.1 per cent solar, a measured radius of 0.14 R ⊙ 2 and a mass of about 12 per cent the mass of the Sun. Although Proxima is considered a moderately active star, its rotation period is ∼ 83 days, 3 and its quiescent activity levels and X-ray luminosity 4 are comparable to the Sun's. New observations reveal the presence of a small planet orbiting Proxima with a minimum mass of 1.3 Earth masses and an orbital period of ∼11.2 days. Its orbital semi-major axis is ∼ 0.05 AU, with an equilibrium temperature in the range where water could be liquid on its surface. 5 The results presented here consist of the analysis of previously obtained Doppler measurements (pre-2016 data), and the confirmation of a signal in a specifically designed follow-up campaign in 2016. The Doppler data comes from two precision radial velocity instruments, both at the European Southern Observatory (ESO): the High Accuracy Radial velocity Planet Searcher (HARPS) and the Ultraviolet and Visual Echelle Spectrograph (UVES). HARPS is a high-resolution stabilized echelle spectrometer installed at the ESO 3.6m telescope (La Silla observatory, Chile), and is calibrated in wavelength using hollow cathode lamps. HARPS has demonstrated radial velocity measurements at ∼1 ms −1 precision over time-scales of years, 6 including on low-mass stars. 7 All HARPS spectra were extracted and calibrated with the standard ESO Data Reduction Software, and radial velocities were measured using a least-squares template matching technique. 7 HARPS data is separated into two datasets. The first set includes all data obtained before 2016 by several programmes (HARPS pre-2016 work, and its value is then used to assess the false-alarm probability (or FAP) of the detection. 14 A FAP below 1% is considered suggestive of periodic variability, and anything below 0.1% is considered to be a significant detection. In the Bayesian framework, signals are first searched using a specialized sampling method 16 that enables exploration of multiple local maxima of the posterior density (the result of this process are the gray lines in Figure 1), and significances are then assessed by obtaining the ratios of evidences of models. If the evidence ratio exceeds some threshold (e.g. B 1 /B 0 > 10 3 ), then the model in the numerator (with one planet) is favoured against the model in the denominator (no planet).A well isolated peak at ∼11.2 days was recovered when analyzing all the night averages in the pre-2016 datasets (Figure 1, panel a). Despite the significance of the signal, the analysis of pre-2016 subsets produced slightly different periods depending on the noise assumptions and which subsets were considered. Confirmation or refutation of this signal at 11.2 days was the main driver for proposing the HARPS PRD campaign. T...

The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto. It resides at present at 95.7 astronomical units (1 AU is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 UT. The event is consistent with a spherical shape for Eris, with radius 1,163 ± 6 kilometres, density 2.52 ± 0.05 grams per cm(3) and a high visible geometric albedo, Pv = 0.96(+0.09)(-0.04). No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ∼1 nanobar, about 10,000 times more tenuous than Pluto's present atmosphere. As Pluto's radius is estimated to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun.

Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies 1,2 . Pluto possesses an atmosphere whereas Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos 3 . Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto 4 , and is currently at a distance to the Sun intermediate between the two. Although Makemake's size (1,420 6 60 km) and albedo are roughly known 5,6 , there has been no constraint on its density and there were expectations that it could have a Plutolike atmosphere 4,7,8 . Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 6 9 km (1s) and 1,502 6 45 km, implying a V-band geometric albedo p V 5 0.77 6 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4-12 nanobar (1s) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 6 0.3 g cm 23 is inferred from the data. Stellar occultations allow detection of very tenuous atmospheres and can provide accurate sizes and albedos 9,10,11,3,12 , so we embarked on a programme of predicting and observing occultations by (136472) Makemake, also known as 2005 FY 9 . The occultation of the faint star NOMAD 1181-0235723 (with magnitude m R 5 18.22, where NOMAD is the Naval Observatory Merged Astronomic Dataset) was predicted in 2010 by methods similar to those used to predict occultations by several large bodies 13 , but refined as shown in Supplementary Information section 1. We arranged a campaign involving 16 telescopes, listed in Supplementary Table 1. The occultation was successfully recorded from seven telescopes, listed in Table 1, at five sites. From the images obtained, we made photometric measurements as a function of time (light curves).The light curves of the occultation are shown in Fig. 1. Fitting synthetic square-well models to the light curves yielded the disappearance and reappearance times of the star (Table 1), from which we calculate one chord in the plane of the sky for each site (see Supplementary Information section 3). On the basis of analyses of the light curves, taking into account the cycle time between the images and the dispersion of the data, we deduce that there were no secondary occultations, so we can reject the existence of a satellite larger than about 200 km in diameter in the areas sampled by the chords. The result is consistent with a deep-image survey that did not find any satellites 16 . The chords can be fitted with two shape models (Fig. 2). Our preferred shape, which is compatible with our own and other observations (see Supplementary Information section 8), corresponds to an elliptical object ...

From time series CCD observations of a fixed and large star field that contained the binary trans-Neptunian object (90482) Orcus (formerly 2004 DW), taken during a period of 33 days, we have been able to derive high-precision relative astrometry and photometry of the Orcus system with respect to background stars. The right ascension residuals of an orbital fit to the astrometric data revealed a periodicity of 9.7 ± 0.3 days, which is what one would expect to be induced by the known Orcus companion (Vanth). The residuals are also correlated with the theoretical positions of the satellite with regard to the primary. We therefore have revealed the presence of Orcus' satellite in our astrometric measurements, although the residuals in declination did not show the expected variations. The oscillation in the residuals is caused by the photocenter motion of the combined Orcus plus satellite system around the barycenter along an orbital revolution of the satellite. The photocenter motion is much larger than the motion of Orcus around the barycenter, and we show here that detecting some binaries through a carefully devised astrometric technique might be feasible with telescopes of moderate size. We discuss the prospects for using the technique to find new binary trans-Neptunian objects (TNOs) and to study already known binary systems with uncertain orbital periods. We also analyzed the system's mid-term photometry in order to determine whether the rotation could be tidally locked to the satellite's orbital period. We found that a photometric variability of 9.7 ± 0.3 days is clear in our data, and is nearly coincident with the orbital period of the satellite. We believe this variability might be induced by the satellite's rotation. In our photometry there is also a slight hint for an additional very small variability in the 10 h range that was already reported in the literature. This short-term variability would indicate that the primary is not tidally locked and therefore the system would not have reached a double synchronous state. Implications for the basic physical properties of the primary and its satellite are discussed. From angular momentum considerations we suspect that the Orcus satellite might have formed from a rotational fission. This requires that the mass of the satellite would be around 0.09 times that of the primary, close to the value that one derives by using an albedo of 0.12 for the satellite and assuming equal densities for both the primary and secondary.

Aims. The goal of the paper is to present new results on light echoes from young stellar objects. Methods. Broad band CCD images were obtained over three months at one-to-two week intervals for the field of NGC 6726, using the large field-of-view remotely-operated telescope on top of Cerro Burek. Results. We detected scattered light echoes around two young, low-amplitude, irregular variable stars. Observations revealed not just one, but multiple light echoes from brightness pulses of the T Tauri star S CrA and the Herbig Ae/Be star R CrA. Analysis of S CrA's recurring echoes suggests that the star is located 138 ± 16 pc from Earth, making these the closest echoes ever detected. The environment that scatters the stellar light from S CrA is compatible with an incomplete dust shell or an inclined torus some 10 000 AU in radius and containing ∼2 × 10 −3 M of dust. The cause of such concentration at ∼10 000 AU from the star is unknown. It could be the remnant of the envelope from which the star formed, but the distance of the cloud is remarkably similar to the nominal distance of the Oort cloud to the Sun, leading us to also speculate that the dust (or ice) seen around S CrA might have the same origin as the Solar System Oort cloud.