In recent years, the media has been filled with news about the thousands of extrasolar planets found by NASA’s Kepler spacecraft as it monitors the brightness of hundreds of thousands of stars looking for transits. Because of the wide net cast by Kepler in its primary and now its “K2” extended mission, most of those exoplanets discoveries are many hundreds of light years away or more. Inevitably the lay public asks “why aren’t astronomers looking for exoplanets around stars closer to us… maybe someplace we might have a chance of reaching someday?”.

The short answer to this question is, they already are! For over two thirds of a century, astronomers have been using a range of increasingly sophisticated and sensitive techniques to search virtually every star within a few dozen light years for orbiting exoplanets (see Drew Ex Machina’s Nearby Star page for articles on these searches). One of the better known ongoing exoplanet surveys of nearby stars has been conducted by a European team of astronomers operating the HARPS (High Accuracy Radial velocity Planet Search) spectrograph attached to the European Southern Observatory’s 3.6-meter telescope in La Silla, Chile. Using precision radial velocity measurements, HARPS is able to detect the subtle variations caused by the reflex motion of the parent star as any planets of sufficient size orbit them.

In a recent paper submitted for publication in the peer-reviewed Astronomy & Astrophysics with Nicola Astudillo-Defru (Observatoire de Genève) as the lead author, the latest analysis results of HARP data for five nearby M-dwarf stars are described in detail. In total, Astudillo-Defru et al. identified a dozen exoplanets in their data including a previously described trio of exoplanets found orbiting Wolf 1061 (one of which is potentially habitable) for which they provide significantly updated parameters (see “Habitable Planet Reality Check: Wolf 1061c Revisited“). Others are described in this paper for the first time. One of the stars identified as GJ 273 by Astudillo-Defru et al. was found to have a pair of exoplanets somewhat more massive than the Earth including one which is claimed to orbit inside of the habitable zone (HZ) of this red dwarf. This would make this new find the second closest such exoplanet known after Proxima Centauri b (see “Habitable Planet Reality Check: Proxima Centauri b”). So is this newly discovered nearby exoplanet potentially habitable?

Background

GJ 273 is a V magnitude 9.87 star located in the constellation of Canis Minor. Frequently referred to by the designation of BD+5° 1668 from the Bonner Durchmusterung catalog compiled during the late 19th century at the Bonn Observatory, this star is also known to many as Luyten’s Star after the Dutch-American astronomer Willem Jacob Luyten (1899-1994) who had spent his career cataloging dim, high proper motion stars. This star first came to the attention of Luyten and his colleague, Edwin G. Ebbighausen (1911-1984), in 1935 because of its high proper motion of 3.7 arc seconds per year. Subsequent measurements of its parallax indicated that it is a nearby star only 12.4 light years away. According to the best data available on GJ 273 compiled by Astudillo-Defru et al., this spectral type M3.5V red dwarf has a radius of 0.293±0.027 times that of the Sun and a surface temperature 3382±49 K. The luminosity is given as 0.0088 times that of the Sun and the mass is estimated to be 0.29 times. All in all, GJ 273 is moderate size red dwarf that is fairly typical of the stars in this corner of the Milky Way.

Like most nearby stars, Luyten’s Star has been the target of exoplanet searches for decades. The most sensitive search results previously published was from an analysis of HARPS radial velocity measurements published in 2013 with Xavier Bonfils (Observatoire de Genève) as the lead author. Their analysis of 49 radial velocity measurements showed clear signs of variations with a range of possible periods including 18½ days and a very long period around 440 days. While the lack of any correlation with various measurements of stellar activity suggested a possible planetary origin for these variations, the poor sampling in phase space resulted in a high false alarm probability for any potential exoplanet orbit solution. More radial velocity data were needed to sort out what was going on with this star.

The new paper by Astudillo-Defru et al. presents the results of the analysis of a significantly expanded HARPS data set. A total of 280 HARPS spectra acquired between December 8, 2003 and September 30, 2016 were analyzed this time – almost six times as much data than was available to Bonfils et al.. A total of 43 spectra out of this 12.8-year data set were acquired after a fiber optic upgrade was introduced in May 2015 which helped to stabilize the line spread function of HARPS data reducing the noise in the derived radial velocity measurements. Periodogram analysis of the new data set once again showed a strong periodicity of ~420 days as well as ~700 days. These peaks have been discounted as potential exoplanet detections because the periods seem to be related to stellar surface activity and aliasing caused by how that activity was sampled during the survey.

On a much firmer footing are signals found with a period of 18.65 and 4.723 days. These signals show no correlation with stellar activity or the presumed 99-day rotation period found by Astudillo-Defru et al. for this star. The best plausible explanation for the observed variations in the radial velocity of Luyten’s Star is the presence of a pair of super-Earth and Earth-mass planets with orbital periods of 18.65 and 4.723 days, respectively. Key properties of the exoplanets found orbiting GJ 273 as determined by Astudillo-Defru et al. are summarized below in Table 1. Also included are the effective stellar flux or S eff values which provide a measure of the total energy these exoplanets receive from their parent star with the Earth’s value defined as being 1.

Table 1: Properties of the Planets of Luyten’s Star or GJ 273

Planet b c M p sini (Earth=1) 2.89 (+0.27/-0.26) 1.18±0.16 Orbit Period (days) 18.65 4.723 Semi Major Axis (AU) 0.09110 0.03647 S eff (Earth=1) 1.06 6.66

Since the inclination, i, of the planets’ orbits with respect to the plane of the sky can not be determined from radial velocity measurements alone, only the minimum mass or M p sini of these exoplanets can be determined at this time. The actual masses will likely be higher than these indicated values.

Potential Habitability

A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets are basic orbit parameters, a rough measure of its size and/or mass and some important properties of its sun. Combined with theoretical extrapolations of the factors that have kept the Earth habitable over billions of years (not to mention why our neighbors are not habitable), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.

The first step in assessing the potential habitability of the planets orbiting GJ 273 is to determine what sort of worlds they are: are they rocky planets like the Earth or are they volatile-rich mini-Neptunes with little prospects of being habitable in a conventional sense. Unfortunately, the only information currently available about these new planets are their minimum mass. The actual mass and the radius are needed to calculate their average density and get a handle on their bulk compositions. Given their tight orbits, direct imaging of these new planets will require a ten-meter class, space-based telescope with an advanced starshade – a piece of hardware that will likely not be available for decades.

There is the possibility that one or both of these exoplanets have their orbits align by random chance to produce transits observable from the Earth. Such transits would allow the planets’ radii and orbit inclination, i, to be determined. Calculations show that there are 1.6% and 4.3% probabilities that GJ 273b and c produce transits, respectively. The detection of the couple of tenths of a percent decrease in brightness caused by any transits of these roughly Earth-size planets could be possible using existing ground-based instruments. Failing this, NASA’s TESS (Transiting Exoplanet Survey Satellite) spacecraft scheduled for launch in March 2018 should be able to spot any transits from these or other exoplanets that might be orbiting GJ 273 if they exist. At least one transit of GJ 273b and as many as six of GJ 273c could be observable during a standard 27.4-day TESS observing cycle.

In lieu of this vital information, statistical arguments can be made about the probability these new planets have a rocky composition. An analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Rogers strongly suggests that the population of known exoplanets transitions from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 times that of the Earth or R E (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). While rocky planets larger than this are possible, they become more uncommon with increasing radius. A planet with a radius of 1.6 R E and an Earth-like composition would have a mass of about 6 times that of the Earth or M E . With their currently unconstrained orbit orientation, there is about a 2% chance that GJ 273c, with a minimum mass of 1.18 M E , exceeds the 6 M E threshold and it is therefore most likely a rocky planet. The probability that GJ 273b, with an M p sini of 2.89 M E , exceeds this threshold is higher at about 12%.

However, before we read too much into this seemingly rosy assessment, more recent work by Chen and Kipping with a larger sample of exoplanets suggests that the gradual transition of the exoplanetary population from predominantly rocky planets to volatile-rich worlds starts at about 2 M E . This much lower threshold for the start of the transition significantly increasing the chances that these worlds, with only their M p sini values known, are mini-Neptunes. Until a more quantitative estimate can be made based on an analysis of the available data, we can only state that it would seem likely that both of these exoplanets are rocky but there is still a fairly substantial, non-zero probability that they are mini-Neptunes instead, especially the larger GJ 273b.

Another important criterion which can be used to determine if a planet is potentially habitable is the amount of energy it receives from its parent star known as the effective stellar flux or S eff . According to the work by Kopparapu et al. (2013, 2014) on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the inner limit of the HZ is conservatively defined by the runaway greenhouse limit where a planet’s temperature would soar even with no CO 2 present in its atmosphere resulting in the loss of all of its water in a geologically brief time in the process. For an Earth-size planet orbiting Luyten’s Star, this happens at an S eff value of 0.92 which corresponds to a mean orbital distance of 0.097 AU. For a more massive 5 M E exoplanet, the S eff for the inner edge of the HZ is 1.00 or a distance of about 0.094 AU due to the compression of the atmosphere caused by the higher surface gravity. With an S eff of 6.66, GJ 273c is clearly well beyond the inner edge of the HZ and is likely to be a larger, hotter version of Venus. On the other hand, GJ 273b with an S eff of 1.06 appears to orbit near the edge of the HZ, depending on its actual mass, M P . The current uncertainties in the properties of Luyten’s Star as well as the orbit of this exoplanet make it appear that there are about even odds that it orbits inside of the conservatively defined HZ.

The probability that GJ 273b orbits inside of the HZ is likely higher than this, however. Because of the tight orbits of these exoplanets, they would be expected to be synchronous rotators with the same side perpetually facing their sun. Detailed climate modeling over the last two decades now shows that synchronous rotation is probably not the impediment to global habitability as it was once thought. In fact, it has been shown that slow or synchronous rotation can actually result in an increase of the S eff for the inner edge of the HZ. According to the recent work by Yang et al., the inner edge of the HZ for a slow rotator orbiting a star like GJ 273 would have an S eff of 1.62 corresponding to an orbital distance of just 0.074 AU. A more recent paper by Kopparapu et al. (2016) which takes into account the effects of short orbital periods on atmospheric circulation also suggests that the S eff value for the inner edge of the HZ for a synchronous rotator of Luyten’s Star would be about 1.64. Even with the uncertainties in the properties of GJ 273b, it would appear that it orbits comfortably inside the HZ for a slow or synchronous rotator. This fact combined with the fair likelihood that GJ 273b has a predominantly rocky composition makes this exoplanet a reasonably good candidate for being a potentially habitable – potentially deserving of being on a future “Top 5” list among exoplanets like Proxima Centauri b, Kepler 186f or Kepler 452b (see “The Top Five Known Potentially Habitable Planets“).

Summary

While the planetary nature of the radial velocity variations of Luyten’s Star found by Astudillo-Defru et al. needs to be independently confirmed, at this time it does appear that one of them, GJ 273b, is a fairly good candidate for being a potentially habitable exoplanet. This exoplanet appears to orbit at the edge of the conservatively defined habitable zone (HZ) and comfortably inside the HZ defined for slow or synchronous rotators. While only the minimum mass of GJ 273b is known, it appears probable that it is a predominantly rocky planet like the Earth although a volatile-rich bulk composition which would preclude Earth-like habitability can not be excluded. Astudillo-Defru et al. also point out that Luyten’s Star is a relatively quiet red dwarf diminishing the potentially erosive effects of flaring on the atmosphere of this exoplanet. In this regard, at least, GJ 273b is a better candidate for being potentially habitable than Proxima Centauri b.

Considering what we do know about this exoplanet, it would appear that GJ 273b is a good candidate for being potentially habitable and is currently the second closest known example after Proxima Centauri b. Although much more needs to be learned about the exoplanets orbiting Luyten’s Star, they promise to be excellent targets for future investigations from the ground and space because of its proximity to the Sun and the star’s relative brightness.

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Related Reading

For a complete collection of articles about our other neighboring star systems and the searches for exoplanets orbiting them, see Drew Ex Machina’s page on Nearby Stars.

General References

N. Astudillo-Defru et al., “The HARPS search for southern extra-solar planets XLI. A dozen planets around the M dwarfs GJ 3138, GJ 3323, GJ 273, GJ 628, and GJ 3293”, arXiv 1703.05386 (accepted by Astronomy & Astrophysics), March 15, 2017 [Preprint]

X. Bonfils et al., “The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample”, Astronomy & Astrophysics, Vol. 549, ID A8, January 2013

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, The Astrophysical Journal, Vol. 834, No. 1, Article id. 17, January 2017

R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

Ravi Kumar Kopparapu et al., “The Inner Edge of the Habitable Zone for Synchronously Rotating Planets around Low-mass Stars Using General Circulation Models”, The Astrophysical Journal, Vol. 819, No. 1, Article ID. 84, March 2016

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015

Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article id. L2, May 2014