Prospects for detecting the astrometric signature of Barnard’s Star b

Tal-Or, L.; Zucker, S.; Ribas, I.; Anglada-Escudé, G.; Reiners, A.

doi:10.1051/0004-6361/201834643

Cited by 8 publications

(5 citation statements)

References 48 publications

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“…The Gaia and HST missions can reach an astrometric accuracy of 0.03 mas 18,19 . Depending on the orbital inclination they could detect the planet signal or set a constraining mass upper limit 20 . Also, for the calculated orbital separation the contrast ratio between the planet and the star in reflected light is of the order of a few times 10 -9 depending on the adopted values of the geometric albedo and orbital inclination.…”

Section: Figure 1 and Edmentioning

confidence: 99%

A candidate super-Earth planet orbiting near the snow line of Barnard’s star

Ribas

Tuomi

Reiners

et al. 2018

Nature

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118

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At a distance of 1.8 parsecs 1 , Barnard's star (Gl 699) is a red dwarf with the largest apparent motion of any known stellar object. It is the closest single star to the Sun, second only to the a Centauri triple stellar system. Barnard's star is also among the least magnetically active red dwarfs known 2,3 and has an estimated age older than our Solar System. Its properties have made it a prime target for planet searches employing techniques such as radial velocity 4,5,6 , astrometry 7,8 , and direct imaging 9 , all with different sensitivity limits but ultimately leading to disproved or null results. Here we report that the combination of numerous measurements from high-precision radial velocity instruments reveals the presence of a low-amplitude but significant periodic signal at 233 days. Independent photometric and spectroscopic monitoring, as well as the analysis of instrumental systematic effects, show that this signal is best explained as arising from a planetary companion. The candidate planet around Barnard's star is a cold super-Earth with a minimum mass of 3.2 Earth masses orbiting near its snow-line. The combination of all radial velocity datasets spanning 20 years additionally reveals a long-term modulation that could arise from a magnetic activity cycle or from a more distant planetary object. Because of its proximity to the Sun, the proposed planet has a maximum angular separation of 220 milliarcseconds from Barnard's star, making it an excellent target for complementary direct imaging and astrometric observations.Barnard's star is the second closest red dwarf to the Solar System, after Proxima Centauri, and thus an ideal target to search for exoplanets with potential for further characterisation 10 . Its very low X-ray flux, lack of Ha emission, low chromospheric emission indices, slow rotation rate, slightly sub-solar metallicity, and membership of the thick disc kinematic population indicate an extremely low magnetic activity level and suggest an age older than the Sun. Because of its apparent brightness and very low variability, Barnard's star is often regarded as a benchmark for intermediate M-type dwarfs. Its basic properties are summarized in Table 1.An early analysis of archival radial velocity datasets of Barnard's star up to 2015 indicated the presence of at least one significant signal with a period of ~230 days but with rather poor sampling. To elucidate its presence and nature we undertook an intensive monitoring campaign with the CARMENES spectrometer 11 , collecting precise radial velocity measurements on every possible night during 2016-2017, and we obtained overlapping observations with the ESO/HARPS and HARPS-N instruments. The combined Doppler monitoring effort of Barnard's star, including archival and newly acquired observations, resulted in 771 radial velocity epochs (nightly averages) with typical individual precisions of 0.9 to 1.8 m s -1 , obtained over a timespan exceeding 20 years from seven different facilities and yielding eight independent datasets (ED Table 1).While e...

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Section: Figure 1 and Edmentioning

confidence: 99%

A candidate super-Earth planet orbiting near the snow line of Barnard’s star

Ribas

Tuomi

Reiners

et al. 2018

Nature

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118

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“…Recently, Ribas et al (2018) announced a super-Earth candidate (Barnard b) of minimum mass 3.23 M ⊕ around this star on an orbit of semi-major axis a = 0.4 AU, period P = 232 days, and maximum angular separation ∆θ = 220 milliarcseconds (mas). This planet has not been observed in transit (Tal-Or et al 2019), so its atmosphere can only be characterized by measuring the light either reflected or emitted by the planet.…”

Section: Introductionmentioning

confidence: 99%

Directly imaged exoplanets in reflected starlight: the importance of knowing the planet radius

Carrión-González

Muñoz

Cabrera

et al. 2020

A&A

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Context. The direct imaging of exoplanets in reflected starlight will represent a major advance in the study of cold and temperate exoplanet atmospheres. Understanding how basic planet and atmospheric properties may affect the measured spectra is key to their interpretation. Aims. We have investigated the information content in reflected-starlight spectra of exoplanets. We apply our analysis to Barnard’s Star b candidate super-Earth, for which we assume a radius 0.6 times that of Neptune, an atmosphere dominated by H2–He, and a CH4 volume mixing ratio of 5 × 10−3. The main conclusions of our study are however planet-independent. Methods. We set up a model of the exoplanet described by seven parameters including its radius, atmospheric methane abundance, and basic properties of a cloud layer. We generated synthetic spectra at zero phase (full disc illumination) from 500 to 900 nm and a spectral resolution R ~ 125–225. We simulated a measured spectrum with a simplified, wavelength-independent noise model at a signal-to-noise ratio of 10. With a retrieval methodology based on Markov chain Monte Carlo sampling, we analysed which planet and atmosphere parameters can be inferred from the measured spectrum and the theoretical correlations amongst them. We considered limiting cases in which the planet radius is either known or completely unknown, and intermediate cases in which the planet radius is partly constrained. Results. If the planet radius is known, we can generally discriminate between cloud-free and cloudy atmospheres, and constrain the methane abundance to within two orders of magnitude. If the planet radius is unknown, new correlations between model parameters occur and the accuracy of the retrievals decreases. Without a radius determination, it is challenging to discern whether the planet has clouds, and the estimates on methane abundance degrade. However, we find the planet radius is constrained to within a factor of two for all the cases explored. Having a priori information on the planet radius, even if approximate, helps improve the retrievals. Conclusions. Reflected-starlight measurements will open a new avenue for characterizing long-period exoplanets, a population that remains poorly studied. For this task to be complete, direct-imaging observations should be accompanied by other techniques. We urge exoplanet detection efforts to extend the population of long-period planets with mass and radius determinations.

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“…Direct imaging with the next generation of large telescopes might help resolve this. R18 states that the separation between the proposed planet and the star would be great enough for Hubble to detect an astrometric signal, which Tal-Or et al (2019b) confirms and goes on to include Gaia DR2 as an instrument capable of making this measurement. If such measurements are undertaken, based on the result we have presented in this paper we would not expect a significant signal corresponding to the planet candidate proposed by R18.…”

Section: Discussionmentioning

confidence: 83%

Stellar Activity Manifesting at a One Year Alias Explains Barnard b as a False Positive

Lubin¹,

Robertson²,

Stefánsson³

et al. 2021

Preprint

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Barnard's star is among the most studied stars given its proximity to the Sun. It is often considered the Radial Velocity (RV) standard for fully convective stars due to its RV stability and equatorial declination. Recently, an M sin i = 3.3M ⊕ super-Earth planet candidate with a 233 day orbital period was announced by Ribas et al. (2018). New observations from the nearinfrared Habitable-zone Planet Finder (HPF) Doppler spectrometer do not show this planetary signal. We ran a suite of experiments on both the original data and a combined original + HPF data set. These experiments include model comparisons, periodogram analyses, and sampling sensitivity, all of which show the signal at the proposed period of 233 days is transitory in nature. The power in the signal is largely contained within 211 RVs that were taken within a 1000 day span of observing. Our preferred model of the system is one which features stellar activity without a planet. We propose that the candidate planetary signal is an alias of the 145 day rotation period. This result highlights the challenge of analyzing long-term, quasi-periodic activity signals over multi-year and multi-instrument observing campaigns.

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Prospects for detecting the astrometric signature of Barnard’s Star b

Cited by 8 publications

References 48 publications

A candidate super-Earth planet orbiting near the snow line of Barnard’s star

A candidate super-Earth planet orbiting near the snow line of Barnard’s star

Directly imaged exoplanets in reflected starlight: the importance of knowing the planet radius

Stellar Activity Manifesting at a One Year Alias Explains Barnard b as a False Positive

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