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The (Historical) Search for Planets Orbiting Proxima Centauri

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red dwarf exoplanet
Artist’s illustration of a planet (with two moons) orbiting in the habitable zone of a red dwarf star. We’ve spent the last couple decades searching for planets around our nearest stellar neighbor, the red dwarf Proxima Centauri. [NASA/Harvard-Smithsonian Center for Astrophysics/D. Aguilar]

The European Southern Observatory (ESO) is widely expected to address the reports of the discovery of a planet orbiting our nearest stellar neighbor, Proxima Centauri, today. Due to its proximity — 4.25 light-years away — this red dwarf star has been a prime target for exoplanet searches throughout the last couple decades.

Proxima Centauri

Hubble image of Proxima Centauri, our nearest stellar neighbor. [ESA/Hubble]

In anticipation of ESO’s press conference this afternoon, let’s take a look at some of the past work in the search for planetary companions around Proxima Centauri.

The Early Years of Exploring Proxima Centauri

Proxima Centauri was discovered by astronomer Robert Innes in 1915. Studies of this star over the next eighty years primarily focused on better understanding its orbital motion (is it part of the Alpha Centauri star system?) and its flaring nature. But in the 1990s, after the detection of the first exoplanets, Proxima Centauri became a target for its potential to host planet-mass companions.

HST FOS Proxima Centauri

Top: Images of Proxima Centauri on two different days from Hubble’s FOS instrument. The bar across the center is an occulter that partially blocks the light from Proxima Centauri. Middle: Reconstructed images allowing a closer look at a moving feature identified by the authors as a possible companion. Bottom: diagram of the position of the planet candidate (box) relative to Proxima Centauri (star) in the two frames. [Schultz et al. 1998]

1990s: A Possible Planet Detected With Hubble?

In January 1998, a paper led by A.B. Schultz (STScI) reported the possible visual detection of a planetary companion to Proxima Centauri. Observations from Hubble’s Faint Object Spectrograph, which was being used as a coronagraphic camera, revealed excess light that could be interpreted as a substellar object located ~0.5 AU from Proxima Centauri, a small separation that could imply either a short (~1 yr) period or a highly eccentric orbit.

But follow-up observations led by David Golimowski (Johns Hopkins University) were unable to detect this proposed planet. These observations — made by direct imaging with Hubble’s Wide Field Planetary Camera 2 — found no evidence of a companion located 0.12–1.1 AU from Proxima Centauri.

In addition, an astrometric study led by G. Fritz Benedict (McDonald Observatory) the following year also didn’t find any evidence for the proposed companion. Along with prior radial velocity measurements, the astrometry in this study ruled out all companions to Proxima Centauri with a mass of more than 0.8 Jupiter masses and periods between 1 and 1000 days.

Increased Capabilities in Recent Years

With increasing resolution and sensitivity of instruments, as well as better stellar modeling and increased noise-reduction strategies, we are now more likely than ever to be able to detect a planet orbiting Proxima Centauri. Therefore, our continued non-detections have been placing ever more stringent limits on the mass and orbital properties of a hypothetical companion.

In 2014, as part of a long-term study of the solar neighborhood, a team led by John Lurie (University of Washington) published the results of a nearly 13-year campaign that used the Cerro Tololo Inter-American Observatory to obtain astrometric measurements for Proxima Centauri. This detailed study ruled out the possibility of Jupiter-mass companions at orbital periods of 2–12 years.

HARPS-TERRA RV measurements

Radial-velocity measurements of Proxima Centauri from a 2012 study using HARPS-TERRA. No “promising signals” of companions were found. [Anglada-Escudé and Butler 2012]

One of the most advanced instruments currently in the radial-velocity planet search is a spectrometer called the High Accuracy Radial velocity Planet Searcher (HARPS), operated by ESO in La Silla Observatory, Chile. In a study from 2012 led by Guillem Anglada-Escudé (Carnegie Institution of Washington), the team described new data analysis algorithms being used with HARPS. The authors used Proxima Centauri as a test case, finding only a very marginal signal with a period of 5.6 days. The signal’s lack of significance led them to conclude that, “unfortunately, no promising signals are yet detected on Proxima Cen.”

These studies — among others — throughout the last couple decades have placed strict limitations on the mass and orbit of a potential planetary companion to our nearest stellar neighbor. It will be interesting to see what ESO announces this afternoon, and how it fits into the context of these past studies of Proxima Centauri!

Citation

A. B. Schultz et al 1998 AJ 115 345. doi:10.1086/300176
David A. Golimowski and Daniel J. Schroeder 1998 AJ 116 440. doi:10.1086/300437
G. Fritz Benedict et al 1999 AJ 118 1086. doi:10.1086/300975
John C. Lurie et al 2014 AJ 148 91. doi:10.1088/0004-6256/148/5/91
Guillem Anglada-Escudé and R. Paul Butler 2012 ApJS 200 15. doi:10.1088/0067-0049/200/2/15