Saturday, October 31, 2015

The Ghost in the Window



Vinesh Rajpaul and colleagues have forcefully challenged the reality of Alpha Centauri Bb, the Earth-mass Hellworld proposed as a companion to one of the two nearest Sun-like stars. This extrasolar candidate was announced three years ago by a team led by Xavier Dumusque. Even using the exquisitely sensitive HARPS spectrograph, Dumusque and colleagues noted that detecting such a lightweight object stretched the limits of the radial velocity method and required complex data analytic approaches.

The announcement of this planet in a familiar, nearby system generated great excitement in the exoplanet community, along with an initially subdued but slowly growing skepticism. Artie Hatzes first advised caution (2012) and then expressed doubt (2013) about the planet’s existence. A team including Michael Endl (2015) and Christoph Bergmann (2015) began an observational program focused on both stars in the Alpha Centauri binary to return a definitive picture supported by independent data. So far, however, they have not reported any conclusions.

In their October takedown, Rajpaul and colleagues rely on exhaustive statistical analyses instead of new observations. They show that the existing HARPS data previously interpreted as a terrestrial planet on a three-day orbit are just an artifact of the modeling approach used by Dumusque and colleagues. According to their abstract: 

The 3.24 day signal observed in the Alpha Cen B data almost certainly arises from the window function (time sampling) of the original data. We show that when stellar activity signals are removed from the RV variations, other significant peaks in the power spectrum of the window function are coincidentally suppressed, leaving behind a spurious yet apparently-significant 'ghost' of a signal that was present in the window function’s power spectrum ab initio.

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In related news, Rodrigo Diaz & colleagues (herafter D15) report that, with more than a decade of HARPS radial velocity measurements in hand, they can confirm only four of the six planets proposed for HD 40307. This system includes a single Sun-like star located a bit farther away than Alpha Centauri, at a distance of 12.8 parsecs (42 light years). Notably, one of the two planets phantomized by D15’s summative analysis (HD 40307 g) was previously imagined as a “potentially habitable Super Earth” – in other words, an object with a minimum mass below 10 Earth masses (10 Mea) orbiting in the system habitable zone (Brasser et al. 2014). By now, however, it seems pretty clear that the maximum planet mass compatible with surface water in the habitable zone is closer to 5 Mea than to 10 Mea. Thus the elision of candidate g, whose minimum mass was supposed to be about 7 Mea, provokes no exobiological regrets.

Despite these two subtractions, HD 40307 persists among the classic examples of compact, low-mass multiplanet systems within 40 parsecs. According to D15, the four robustly detected planets follow circular orbits within 0.25 astronomical units (AU) of the star, and their minimum masses range from 3.6 to 8.7 Mea. A dynamical integration of system elements over half a million years indicated that the true masses of these planets could be at least twice their minimum masses without compromising long-term stability. Similar architectures are quite common in the large sample of Kepler multiplanet systems.

Meanwhile, the Sun’s back yard is getting crowded with ghosts! 

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REFERENCES
Bergmann C,.Endl M, Hearnshaw JB, Wittenmyer RA, Wright DJ. (2015) Searching for Earth-mass planets around Alpha Centauri: Precise radial velocities from contaminated spectra. International Journal of Astrobiology 14, 173-176. Abstract: 2015IJAsB..14..173B
Brasser R, Ida S, Kokubo E. (2014) A dynamical study on the habitability of terrestrial exoplanets II: The super Earth HD 40307 g. Monthly Notices of the Royal Astronomical Society 440, 3685-3700. Abstract: 2014MNRAS.440.3685B
Diaz RF, S├ęgransan D, Udry S, Lovis C, Pepe F, Dumusque X, and 19 others. (2015) The HARPS search for southern extra-solar planets XXXVII. Bayesian re-analysis of three systems: New super-Earths, unconfirmed signals, and magnetic cycles. Astronomy & Astrophysics, in press. Abstract: 2015arXiv151006446D
Dumusque X, Pepe F, Lovis C, S├ęgransan D, Sahlmann J, Benz W, Bouchy F, Mayor M, Queloz D, Santos N, Udry S. (2012) An Earth-mass planet orbiting Alpha Centauri B. Nature 491, 207-211. Abstract: http://adsabs.harvard.edu/abs/2012Natur.491..207D
Endl M, Bergmann C, Hearnshaw J, Barnes SI, Wittenmyer RA, Ramm D, Kilmartin P, Gunn F, Brogt E. (2015) The Mt. John University Observatory search for Earth-mass planets in the habitable zone of Alpha Centauri. International Journal of Astrobiology 14, 305-312. Abstract: 2015IJAsB..14..305E
Hatzes A. (2012) Meet Our Closest Neighbour. Nature 491, 200-201.
Hatzes AP. (2013) Radial velocity detection of Earth-mass planets in the presence of activity noise: The case of Alpha Centauri Bb. Abstract: 2013ApJ...770..133H
Rajpaul V, Aigrain S, Roberts S. (2015) Ghost in the time series: No planet for Alpha Cen B. Monthly Notices of the Royal Astronomical Society, in press. Abstract: 2015arXiv151005598R

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