Our nearest neighbor, Proxima Centauri, hosts a temperate terrestrial planet. We detected in radial velocities evidence of a possible second planet with minimum massm
csin (i(c) **************** (=5.8 ± 1.9) ************** (M)
⊕and orbital period**************
************************
************************ (P) **************** () *********************** (c) **************************=(******************************** (5.) ************************ (******************************* (0.) ************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************** (**********************************
**************************** ( ) (0.) **************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
**********************(************************************ 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. () ****************************************** (INTRODUCTION
**********************(************************************ 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. () ****************************************** (INTRODUCTION
(Over more than) ************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************ years, our nearest stellar neighbor Proxima Centauri (GJ 728; hereafter Proxima) has been observed with different techniques aimed at the detection of planetary companions. Proxima is an M5.5V star 1. (± 0.) ******************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************** (pc away from the Sun)1
; Therefore, it is an ideal target for astrometric (
*************************************** (and direct imaging)3
searches. To date, these methods excluded the existence of Jupiter mass planets from 0.8 astronomical unit (AU) to farther than 5 AU (>2 AU for masses
) m (M 4M) *************** (Jupiter) ) ((2) ************ (*********************************************,3
**********************************, ************************************************4
. Holman and Wiegert (5
[𝒰(20–6500) days]. ************************************ predicted a maximum stable orbital radius of AU for planets orbiting Proxima, because the star orbits the double system αCen AB, as has been demonstrated with a high degree of confidence in (********************************
6
(********************************************, (**************** 7) *****************). More recently, Kervella et al
set a 1σ upper limit of 0.3 MJupiterto potential companions of Proxima up to AU by analyzing its proper motion taken from the Gaia Data Release 2 (DR2) and excluded the presence of planets between 15 and (AU in the mass range 0.3 to 8 M) ************** (Jupiter) . Using the radial velocity (RV) technique, Endl and Kürster (
and orbital periods out to days. It was because of the RV technique that the temperate, low-mass planet Proxima b, orbiting at a distance of ∼0. 0006 AU, was discovered (**********************************
9. M dwarfs have high occurrence rates of small planets (1.0 to 2.8R
⊕, 3.5 times more than main-sequence FGK stars (
**********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************); Therefore, systems with multiple, small, low-mass planets are expected to be common around them. With their RV dataset, including measurements with the HARPS (High Accuracy Radial Velocity Planet Searcher) and UVES (Ultraviolet and Visual Echelle Spectrograph) spectrographs, Anglada-Escudéet al
. ( ()
(9) ********** () ************************************ could not rule out the presence of an additional super-Earth in the system with orbital periods longer than that of Proxima b and with Doppler semi-amplitude smaller than 3 ms (1) . For the sake of an independent confirmation of the existence of Proxima b and to search for additional low-mass companions, Damasso and Del Sordo (**************************************) analyzed the same RV measurements using a model that treats the imprint of the stellar activity in a different way than the method adopted in (9
. This analysis uncovered correlated variability in the RV data ascribable to the stellar activity and modulated over the known stellar rotation period. By treating the stellar activity signal with a quasi-periodic model, they could not unambiguously detect a low-mass companion with an orbital period longer than that of Proxima b. The search for additional planets in this system did not stop, and it was at the origin of the Red Dots (RD) initiative (********************************** https: // reddots. space /), which also focused on other nearby stars, and recently led to the discovery of a candidate super-Earth orbiting Barnard’s star close to the snowline ( () ************** 26). Because of the RD campaign, additional RVs of Proxima were collected with the HARPS spectrograph, extending the time span by 000592 days with respect to the dataset analyzed in ( ()(9) ********** (). ***************************************
In this work, we present the results from the analysis of the extended RV dataset carried out within the framework outlined in () and aimed at searching for additional low-mass planetary companions to Proxima. The conclusions of this study are supported by the analysis of spectroscopic activity diagnostics and of a photometric light curve with a baseline longer than the time span of the RV dataset. (**************************************** (***************************************** (MATERIALS AND METHODS)
RV extraction
The RVs from HARPS spectra were extracted using the TERRA (Template-Enhanced Radial velocity Re-analysis Application (pipeline)
and represent an updated dataset with respect to that published in (
************************************************************ ()(9) **********. As in previous works, all observations taken before (various programs) and after [including the Pale Red Dot 2016 (9) and Red Dots 2017 campaigns] the HARPS fiber upgrade in May 2015 were treated as coming from a separate instrument to account for the reported offset introduced by the fiber change. HARPS / ESO (European Southern Observatory) reduced spectra have a known “residual” systematic effect with a ~ 1-year periodicity caused by a small pixel size difference every 551 pixels on the detector, often called the “stitching problem,” coupled to the barycentric motion of the Earth, which implies that some spectral lines go across these pixels (
. As detailed in (
), we masked ± 45 pixels around each of these 549 stitches and rerun the RV velocity measurements for both pre- and post-fiber upgrade datasets. Despite the fact that this removes about (********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************% of the useful Doppler pixels, a bit higher random noise is desirable compared to systematic excursions beating with the yearly sampling. All barycentric corrections were applied as in ( (******************), and secular geometric acceleration was also removed from the final RVs using the known astrometry of the star (DR2). Because we are interested in testing for the presence of longer period signals, we computed nightly weighted means and added 1 ms – 1in quadrature to the formal errors given by the pipeline to account for some unrealistically small uncertainties in some high signal-to-noise (S / N) spectra.
The RV dataset extracted from the UVES spectra, collected between and (*************************************************************************************************************************************************************************************************************************************************************************************************************************************, is an improved version of that used in ( (**************** (9) ****************
), obtained after changes have been applied to the pipeline for processing all the spectra homogeneously. We reanalyzed the entire dataset starting from the raw images and using the associated calibration frames. We reduced the raw images to one-dimensional spectra with our custom reduction package and generated velocities from our precision velocity package. After this full reduction, the root mean square (RMS) of the nightly binned RVs of Proxima has been reduced from 2. (to 2.) *********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************** (m / s)
*************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
. The final dataset that we analyzed consists of (HARPS RVs) before the fiber upgrade) and nightly binned UVES RVs, with a time span of (days.) ************************************** (******************************************************************** Photometry ********************************************************************
In this work, we used a long time span dataset of ASAS -3 and ASAS-4 V-band observations ( () ************** 30
. The light curve was an extension of that shown in ( Fig. 3
) (
), with a time span increased by 1343 days. We used magnitudes measured with the photometric aperture-labeled MAG2 (appropriate to brightness and crowding of the field) in the ASAS data file and considered only high-quality data (flags A and B). Last, we binned the data on a nightly basis. Their dispersion is 0. 046 mag, and the median uncertainty is 0.0 049 mag. The data are listed in table S2. (
Spectroscopic activity diagnostics (******************************************************************************************** (In addition to photometry, we inspected activity indicators extracted from spectra as the full width at half maximum ( only for HARPS) and those based on the chromospheric CaII H K and Hα emission lines. Because the S / N corresponding to CaII H K (as measured from HARPS spectra) is generally less than 1 (median value, 0.5), we selected only the Hα index for a reliable analysis. A key point to address to search for long-term, periodic modulations due to activity (to be eventually compared with any significant long-period signal found in the RVs) is to deal with indexes extracted homogeneously both from the UVES and HARPS spectra, covering the whole time span of the observations. To do so, we used the UVES activity indexes already published in (9) **************, and then we extracted the Hα index from HARPS spectra following the recipe used in (
9
[astro-ph.EP] ************************************** by adapting the code ACTIN). The time series of the Hα index measured from HARPS spectra is listed in table S3.We excluded outliers potentially due to powerful flares through a 3σ clipping of the data, resulting in two epochs removed from the UVES dataset and three epochs removed from the HARPS dataset, therefore with a very limited impact on the analysis. Then, we binned the Hα index extracted from the HARPS spectra on a nightly basis. Last, because there is not one-to-one correspondence between the Hα index and RV datasets (some of the latter missing because the corresponding spectra were discarded by the TERRA pipeline), we searched for and selected those epochs that are in common between the two time series to perform a correct correlation study (this caused 10 RVs to be excluded from the correlation analysis).
Although we used the same recipe fo r both UVES and HARPS, we noted that an offset between the two datasets still exists by comparing the Hα values taken at a similar epoch (BJD=2, (**************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************, 234) and at two consecutive epochs (HARPS, BJD=2,
, 813; UVES, BJD=2, 482, 1306). It is reasonable to expect an instrumental offset when combining data extracted with the same method from spectra collected with different instruments. To produce a complete UVES HARPS dataset free from offset, we subtracted the average value 1. from the UVES Hα index dataset, as determined from measurements at the epochs indicated above. (****************************************
******************************** (RESULTS
We analyzed the enlarged RV dataset spanning ~ 19 years by performing Monte Carlo (MC) analyzes in a Bayesian framework using models based on Gaussian process (GP) regression, as described in detail in the Supplementary Materials. Initially, our model included only the orbital equation of the planet Proxima b combined with the GP term describing the stellar activity contribution to the RVs. The best-fit values of the one-planet model parameters are shown in table S1. Then, we subtracted from the complete RV time series the best-fit solution for planet b (eccentric orbit), a secular acceleration term, and the RV offsets (thus, without removing any activity-related signal), and we analyzed the generalized Lomb -Scargle (GLS) periodogram (of the RV residuals. We found a clear peak with the highest power atP
∼ days, with a false alarm probability of 0. (****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************% as derived by a bootstrap (with replacement) analysis of 13, (randomly generated RV samples)
Fig. 1 (**********************************************************************************************************************************
⊕, 3.5 times more than main-sequence FGK stars (
**********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************); Therefore, systems with multiple, small, low-mass planets are expected to be common around them. With their RV dataset, including measurements with the HARPS (High Accuracy Radial Velocity Planet Searcher) and UVES (Ultraviolet and Visual Echelle Spectrograph) spectrographs, Anglada-Escudéet al
. ( ()
(9) ********** () ************************************ could not rule out the presence of an additional super-Earth in the system with orbital periods longer than that of Proxima b and with Doppler semi-amplitude smaller than 3 ms (1) . For the sake of an independent confirmation of the existence of Proxima b and to search for additional low-mass companions, Damasso and Del Sordo (**************************************) analyzed the same RV measurements using a model that treats the imprint of the stellar activity in a different way than the method adopted in (9
. This analysis uncovered correlated variability in the RV data ascribable to the stellar activity and modulated over the known stellar rotation period. By treating the stellar activity signal with a quasi-periodic model, they could not unambiguously detect a low-mass companion with an orbital period longer than that of Proxima b. The search for additional planets in this system did not stop, and it was at the origin of the Red Dots (RD) initiative (********************************** https: // reddots. space /), which also focused on other nearby stars, and recently led to the discovery of a candidate super-Earth orbiting Barnard’s star close to the snowline ( () ************** 26). Because of the RD campaign, additional RVs of Proxima were collected with the HARPS spectrograph, extending the time span by 000592 days with respect to the dataset analyzed in ( ()(9) ********** (). ***************************************
In this work, we present the results from the analysis of the extended RV dataset carried out within the framework outlined in () and aimed at searching for additional low-mass planetary companions to Proxima. The conclusions of this study are supported by the analysis of spectroscopic activity diagnostics and of a photometric light curve with a baseline longer than the time span of the RV dataset. (**************************************** (***************************************** (MATERIALS AND METHODS)
RV extraction
The RVs from HARPS spectra were extracted using the TERRA (Template-Enhanced Radial velocity Re-analysis Application (pipeline)
and represent an updated dataset with respect to that published in (
************************************************************ ()(9) **********. As in previous works, all observations taken before (various programs) and after [including the Pale Red Dot 2016 (9) and Red Dots 2017 campaigns] the HARPS fiber upgrade in May 2015 were treated as coming from a separate instrument to account for the reported offset introduced by the fiber change. HARPS / ESO (European Southern Observatory) reduced spectra have a known “residual” systematic effect with a ~ 1-year periodicity caused by a small pixel size difference every 551 pixels on the detector, often called the “stitching problem,” coupled to the barycentric motion of the Earth, which implies that some spectral lines go across these pixels (
. As detailed in (
), we masked ± 45 pixels around each of these 549 stitches and rerun the RV velocity measurements for both pre- and post-fiber upgrade datasets. Despite the fact that this removes about (********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************% of the useful Doppler pixels, a bit higher random noise is desirable compared to systematic excursions beating with the yearly sampling. All barycentric corrections were applied as in ( (******************), and secular geometric acceleration was also removed from the final RVs using the known astrometry of the star (DR2). Because we are interested in testing for the presence of longer period signals, we computed nightly weighted means and added 1 ms – 1in quadrature to the formal errors given by the pipeline to account for some unrealistically small uncertainties in some high signal-to-noise (S / N) spectra.
The RV dataset extracted from the UVES spectra, collected between and (*************************************************************************************************************************************************************************************************************************************************************************************************************************************, is an improved version of that used in ( (**************** (9) ****************
), obtained after changes have been applied to the pipeline for processing all the spectra homogeneously. We reanalyzed the entire dataset starting from the raw images and using the associated calibration frames. We reduced the raw images to one-dimensional spectra with our custom reduction package and generated velocities from our precision velocity package. After this full reduction, the root mean square (RMS) of the nightly binned RVs of Proxima has been reduced from 2. (to 2.) *********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************** (m / s)
*************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
. The final dataset that we analyzed consists of (HARPS RVs) before the fiber upgrade) and nightly binned UVES RVs, with a time span of (days.) ************************************** (******************************************************************** Photometry ********************************************************************
In this work, we used a long time span dataset of ASAS -3 and ASAS-4 V-band observations ( () ************** 30
. The light curve was an extension of that shown in ( Fig. 3
) (
), with a time span increased by 1343 days. We used magnitudes measured with the photometric aperture-labeled MAG2 (appropriate to brightness and crowding of the field) in the ASAS data file and considered only high-quality data (flags A and B). Last, we binned the data on a nightly basis. Their dispersion is 0. 046 mag, and the median uncertainty is 0.0 049 mag. The data are listed in table S2. (
Spectroscopic activity diagnostics (******************************************************************************************** (In addition to photometry, we inspected activity indicators extracted from spectra as the full width at half maximum ( only for HARPS) and those based on the chromospheric CaII H K and Hα emission lines. Because the S / N corresponding to CaII H K (as measured from HARPS spectra) is generally less than 1 (median value, 0.5), we selected only the Hα index for a reliable analysis. A key point to address to search for long-term, periodic modulations due to activity (to be eventually compared with any significant long-period signal found in the RVs) is to deal with indexes extracted homogeneously both from the UVES and HARPS spectra, covering the whole time span of the observations. To do so, we used the UVES activity indexes already published in (9) **************, and then we extracted the Hα index from HARPS spectra following the recipe used in (
9
[astro-ph.EP] ************************************** by adapting the code ACTIN). The time series of the Hα index measured from HARPS spectra is listed in table S3.We excluded outliers potentially due to powerful flares through a 3σ clipping of the data, resulting in two epochs removed from the UVES dataset and three epochs removed from the HARPS dataset, therefore with a very limited impact on the analysis. Then, we binned the Hα index extracted from the HARPS spectra on a nightly basis. Last, because there is not one-to-one correspondence between the Hα index and RV datasets (some of the latter missing because the corresponding spectra were discarded by the TERRA pipeline), we searched for and selected those epochs that are in common between the two time series to perform a correct correlation study (this caused 10 RVs to be excluded from the correlation analysis).
Although we used the same recipe fo r both UVES and HARPS, we noted that an offset between the two datasets still exists by comparing the Hα values taken at a similar epoch (BJD=2, (**************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************, 234) and at two consecutive epochs (HARPS, BJD=2,
, 813; UVES, BJD=2, 482, 1306). It is reasonable to expect an instrumental offset when combining data extracted with the same method from spectra collected with different instruments. To produce a complete UVES HARPS dataset free from offset, we subtracted the average value 1. from the UVES Hα index dataset, as determined from measurements at the epochs indicated above. (****************************************
******************************** (RESULTS
We analyzed the enlarged RV dataset spanning ~ 19 years by performing Monte Carlo (MC) analyzes in a Bayesian framework using models based on Gaussian process (GP) regression, as described in detail in the Supplementary Materials. Initially, our model included only the orbital equation of the planet Proxima b combined with the GP term describing the stellar activity contribution to the RVs. The best-fit values of the one-planet model parameters are shown in table S1. Then, we subtracted from the complete RV time series the best-fit solution for planet b (eccentric orbit), a secular acceleration term, and the RV offsets (thus, without removing any activity-related signal), and we analyzed the generalized Lomb -Scargle (GLS) periodogram (of the RV residuals. We found a clear peak with the highest power atP
∼ days, with a false alarm probability of 0. (****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************% as derived by a bootstrap (with replacement) analysis of 13, (randomly generated RV samples)
Fig. 1 (**********************************************************************************************************************************
In this work, we present the results from the analysis of the extended RV dataset carried out within the framework outlined in () and aimed at searching for additional low-mass planetary companions to Proxima. The conclusions of this study are supported by the analysis of spectroscopic activity diagnostics and of a photometric light curve with a baseline longer than the time span of the RV dataset. (**************************************** (***************************************** (MATERIALS AND METHODS)
RV extraction
The RVs from HARPS spectra were extracted using the TERRA (Template-Enhanced Radial velocity Re-analysis Application (pipeline)
and represent an updated dataset with respect to that published in (
************************************************************ ()(9) **********. As in previous works, all observations taken before (various programs) and after [including the Pale Red Dot 2016 (9) and Red Dots 2017 campaigns] the HARPS fiber upgrade in May 2015 were treated as coming from a separate instrument to account for the reported offset introduced by the fiber change. HARPS / ESO (European Southern Observatory) reduced spectra have a known “residual” systematic effect with a ~ 1-year periodicity caused by a small pixel size difference every 551 pixels on the detector, often called the “stitching problem,” coupled to the barycentric motion of the Earth, which implies that some spectral lines go across these pixels (
. As detailed in (
), we masked ± 45 pixels around each of these 549 stitches and rerun the RV velocity measurements for both pre- and post-fiber upgrade datasets. Despite the fact that this removes about (********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************% of the useful Doppler pixels, a bit higher random noise is desirable compared to systematic excursions beating with the yearly sampling. All barycentric corrections were applied as in ( (******************), and secular geometric acceleration was also removed from the final RVs using the known astrometry of the star (DR2). Because we are interested in testing for the presence of longer period signals, we computed nightly weighted means and added 1 ms – 1in quadrature to the formal errors given by the pipeline to account for some unrealistically small uncertainties in some high signal-to-noise (S / N) spectra.
The RV dataset extracted from the UVES spectra, collected between and (*************************************************************************************************************************************************************************************************************************************************************************************************************************************, is an improved version of that used in ( (**************** (9) ****************
), obtained after changes have been applied to the pipeline for processing all the spectra homogeneously. We reanalyzed the entire dataset starting from the raw images and using the associated calibration frames. We reduced the raw images to one-dimensional spectra with our custom reduction package and generated velocities from our precision velocity package. After this full reduction, the root mean square (RMS) of the nightly binned RVs of Proxima has been reduced from 2. (to 2.) *********************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************** (m / s)
*************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
. The final dataset that we analyzed consists of (HARPS RVs) before the fiber upgrade) and nightly binned UVES RVs, with a time span of (days.) ************************************** (******************************************************************** Photometry ********************************************************************
In this work, we used a long time span dataset of ASAS -3 and ASAS-4 V-band observations ( () ************** 30
. The light curve was an extension of that shown in ( Fig. 3
) (
), with a time span increased by 1343 days. We used magnitudes measured with the photometric aperture-labeled MAG2 (appropriate to brightness and crowding of the field) in the ASAS data file and considered only high-quality data (flags A and B). Last, we binned the data on a nightly basis. Their dispersion is 0. 046 mag, and the median uncertainty is 0.0 049 mag. The data are listed in table S2. (
9
[astro-ph.EP] ************************************** by adapting the code ACTIN). The time series of the Hα index measured from HARPS spectra is listed in table S3.We excluded outliers potentially due to powerful flares through a 3σ clipping of the data, resulting in two epochs removed from the UVES dataset and three epochs removed from the HARPS dataset, therefore with a very limited impact on the analysis. Then, we binned the Hα index extracted from the HARPS spectra on a nightly basis. Last, because there is not one-to-one correspondence between the Hα index and RV datasets (some of the latter missing because the corresponding spectra were discarded by the TERRA pipeline), we searched for and selected those epochs that are in common between the two time series to perform a correct correlation study (this caused 10 RVs to be excluded from the correlation analysis).
Although we used the same recipe fo r both UVES and HARPS, we noted that an offset between the two datasets still exists by comparing the Hα values taken at a similar epoch (BJD=2, (**************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************, 234) and at two consecutive epochs (HARPS, BJD=2,
We analyzed the enlarged RV dataset spanning ~ 19 years by performing Monte Carlo (MC) analyzes in a Bayesian framework using models based on Gaussian process (GP) regression, as described in detail in the Supplementary Materials. Initially, our model included only the orbital equation of the planet Proxima b combined with the GP term describing the stellar activity contribution to the RVs. The best-fit values of the one-planet model parameters are shown in table S1. Then, we subtracted from the complete RV time series the best-fit solution for planet b (eccentric orbit), a secular acceleration term, and the RV offsets (thus, without removing any activity-related signal), and we analyzed the generalized Lomb -Scargle (GLS) periodogram (of the RV residuals. We found a clear peak with the highest power atP
∼ days, with a false alarm probability of 0. (****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************************% as derived by a bootstrap (with replacement) analysis of 13, (randomly generated RV samples)
Fig. 1 (**********************************************************************************************************************************
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