Habitable Planet Reality Check 55 Cancri f

Six years ago the discovery of the fifth planet known to orbit 55 Cancri A (abbreviated 55 Cnc A and also known as ρ¹ Cancri A) was officially announced [1]. The possible existence of 55 Cnc f, as it is called, had been discussed in astronomical conferences since 2005 based on radial velocity survey data but it took an additional two years of data and analysis to confirm the result for peer-reviewed publication. And as soon as the discovery of 55 Cnc f was officially announced, the claim was made that this was the first planet to be found in the habitable zone (HZ) of another star [2]. While 55 Cnc f had a mass about half that of Saturn and was surely a gas giant incapable of supporting life as we know it, any large moons it might possess could potentially be habitable.

A couple of weeks ago, a detailed analysis of the radial velocity measurements and transiting timing measurements of the planetary system of 55 Cnc A performed by a team led by Benjamin Nelson (Pennsylvania State University and University of Florida) was released [3]. This analysis, which provides the best calculated values of the mass and orbits of the planets in the 55 Cnc system, along with more recent models of the limits of the HZ, allows us to revisit the claim of the habitability of any potential moons of 55 Cnc f to determine how accurate it was.

Background

The star 55 Cnc A is a K0 type dwarf star located 40.2 light years away in the constellation of Cancer. It is the larger member of a widely spaced binary star system with a projected separation of about 1065 AU between 55 Cnc A and its dimmer M-dwarf companion, 55 Cnc B (also known as Giclas 47-9). 55 Cnc A has an estimated mass of about 91% of the Sun’s and 58% of its luminosity so it is slightly cooler, lighter, and dimmer than our Sun [4]. But with an estimated age of 10.2±2.5 billion years, Cnc 55 A is about twice as old.

Diagram comparing the orbits of the inner planets of our solar system (top) and those of 55 Cnc A (bottom). (Center for Exoplanets and Habitable Worlds – Pennsylvania State University)

55 Cnc A was among the first main sequence stars to have planets discovered orbiting it with indications of their existence first discussed publicly in April 1996 [5] and officially announced the following year [6]. Over the next decade, a total of five planets were discovered around this star using the radial velocity technique ranging from a transiting super-Earth orbiting just 1.5 million kilometers above the surface of 55 Cnc A to a distant gas giant in a Jupiter-like orbit 817 million kilometers away. But determining the precise properties of these planets and their orbits has been a daunting task.

Radial velocity measurements alone, while they can reveal information about orbits, can only give the minimum mass of the planets because the inclinations of the planets’ orbit to our line of sight is unknown. The exception in this system is the innermost planet, 55 Cnc e, which transits its sun once each orbit allowing its inclination to be determined. Two of the planets, 55 Cnc b and c, are in a near 3:1 resonance which strongly perturbs their orbits in a periodic way in addition to affecting the orbits of the other planets nearby. Because of this strong coupling between the orbits of these planets it is possible, in theory, to determine the orbits and masses of the planets in this system by fitting the available observations with dynamical simulations of the system. Unfortunately the number of computations required to do this sort of analysis is astronomical (no pun intended) and studies to date on the dynamics of this planetary system have been more limited in scope.

The latest work by Nelson et al. [3] got around this computational bottleneck by pioneering a new technique that uses high-speed computer graphics cards to perform the required calculations for data fits and dynamical simulations that run up to four or five times faster than would be otherwise possible. For their work, they combined 1,418 radial velocity observations acquired over two decades from four different observatories and transit timing data derived from photometric observations from the Canadian MOST (Microvariability and Oscillations of STars) micro satellite launched in 2003. They were then able to derive a self consistent dynamical model that best fits the available observations and remains stable over time frames of at least 100 million years. The properties of the 55 Cnc A planetary system (derived primarily from this paper) are summarized below in Table I.

Table I: Properties of Planets in 55 Cancri A System

Habitability of 55 Cnc f

These latest results provide the best estimates of the properties of the 55 Cnc A planetary system that we currently have. While the results do refine the estimated mass somewhat, 55 Cnc f is still about 47 times more massive than the Earth and is almost certainly a gas giant – smaller than Saturn but larger than Neptune. The new estimate for its orbit combined with the latest measurements of the properties of 55 Cnc A [4] allow us to determine the effective stellar flux, S_eff, or insolation of 55 Cnc f as 0.97 or slightly less than that of Earth’s (which is defined as S_eff = 1). Taking into account the 5196 K effective temperature of 55 Cnc A (which is lower than the Sun’s 5780 K and affects the location of the HZ), the best conservative models for the inner limits of the HZ estimate an S_eff of 0.97 for the onset of the moist greenhouse effect where surface temperatures start to rise and a planet or moon begins to lose its water on time scales of maybe hundreds of millions of years [7]. A runaway greenhouse effect, where temperatures quickly skyrocket to uninhabitable levels, sets in at S_eff of about 0.98.

Since this model of the inner limit of the HZ does not take into account the feedback mechanisms from changes in cloud cover as a function of increasing insolation, it is likely that the actual S_eff of the inner edge of the HZ is slightly higher than this. But given the additional uncertainties in the properties of 55 Cnc A and f, it seems that the best that can be said is that 55 Cnc f is right on the inner edge of the HZ. So it would seem that any sufficiently large moon of 55 Cnc f would be right on the cusp of being habitable, the other potential issues with habitable moons notwithstanding [8]. However, 55 Cnc A is already a fairly old star and it would have been a significant fraction dimmer during its early life just like all other main sequence stars including the Sun. As a result, 55 Cnc f should have been comfortably inside the HZ during most of its existence and only now is leaving (or about to leave) the HZ as 55 Cnc A continues to brighten with age.

While any potentially habitable moon of 55 Cnc f is on the verge of becoming uninhabitable, what about the possibility of other planets further out into the HZ of 55 Cnc A? There is a 4.7 AU wide gap between the orbits of 55 Cnc f and d and at first blush there would seem to be plenty of room for additional planets including more Earth-like terrestrial planets which could easily escape detection using the currently available measurements. One study has shown that a planet orbiting in a zone extending from about 0.9 to 3.8 AU would have a stable orbit so long as its orbital eccentricity was less than ~0.4 [9]. There should be sufficient room for two or three planets in this zone so long as their masses are less than about 50 times that of the Earth (if they were more massive, they would be detected by radial velocity surveys and would destabilize each others’ orbits).

This study was performed before the work by Nelson et al. and their assumed parameters for 55 Cnc f and d were slightly different but not enough to significantly affect their conclusions. With the conservative outer edge of the HZ defined by the maximum greenhouse limit, the outer edge of the HZ of 55 Cnc A would have a S_eff of 0.31 which is about 1.36 AU from 55 Cnc A [7]. While there might be some unresolved issues about the ability of an additional planet to form between 55 Cnc f and d, there would seem to be sufficient room from a dynamical point of view for there to be potentially habitable, possibly terrestrial, planet orbiting 55 Cnc A in the HZ beyond 55 Cnc f.

Conclusion

So once again, I readily admit that I went into this exercise with the aim of disproving the original claim that 55 Cnc f was inside the HZ. But based on the latest measurements of the properties of the 55 Cnc A planetary system and conservative models for the limits of the HZ, it would appear that 55 Cnc f is right on the inner edge of the HZ where a habitable planet or moon would start to lose it vital supply of water and then experience a runaway greenhouse effect. While 55 Cnc f is almost certainly a gas giant that can not support life as we know it and any hypothetical large moon it might have would be on the verge of becoming uninhabitable today (or possibly already started to become uninhabitable), during much of the estimated 10 billion year life of 55 Cnc A, any such moon would have been comfortably inside the HZ until only recently. And given the known properties of the 55 Cnc A system, the possibility exists that another, more distant planet that has yet to be discovered could currently reside in a more distant portion of the HZ.

Related Reading

“Extrasolar Planet Update”, SETIQuest, Vol. 2, No. 4, pp. 6-11, 1996 [Article]

“The Extremes of Habitability”, SETIQuest, Volume 4, Number 2, pp. 1-8, Second Quarter 1998 [Article]

“Habitable Moons: A New Frontier for Exobiology”, SETIQuest, Volume 3, Number 1, pp. 8-16, First Quarter 1997 [Article]

“Habitable Moons”, Sky & Telescope, Volume 96, Number 6, pp. 50-56, December 1998 [On line version]

“Habitable Planet Reality Check: Kepler 186f”, Drew Ex Machina, April 20, 2014 [Post]

References

(1) Debra A. Fischer, Geoffrey W. Marcy, R. Paul Butler, Steven S. Vogt, Greg Laughlin, Gregory W. Henry, David Abouav, Kathryn M. G. Peek, Jason T. Wright, John A. Johnson, Chris McCarthy, Howard Isaacson, “Five Planets Orbiting 55 Cancri”, The Astrophysical Journal, Vol. 675, No. 1, pp. 790–801, March 2008

(2) Shige Abe, “Researchers Identify First Five-Planet Extrasolar System”, Astrobiology: Life in the Universe, NASA, posted November 12, 2007 [Link]

(3) B. Nelson et al. “The 55 Cancri Planetary System: Fully Self-Consistent N-body Constraints and a Dynamical Analysis”, Monthly Notices of the Royal Astronomical Society, Advance Access, 10 pp., April 2014 [Abstract & Paper Access]

(4) Kaspar von Braun et al., “55 Cancri: Stellar Astrophysical Parameters, a Planet in the Habitable Zone, and the Implications for the Radius of a Transiting Planet”, The Astrophysical Journal, Vol. 740, No. 1, Article ID. 49, October 2011

(5) Andrew J. LePage, “Extrasolar Planet Update”, SETIQuest, Vol. 2, No. 4, pp. 6-11, 1996 [Article]

(6) Paul Butler et al., “Three New 51 Pegasi Type Planets”, The Astrophysical Journal Letters, Vol. 474, No. 2, L115–L118, January 1997

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

(8) Andrew J. LePage, “Habitable Moons: A New Frontier for Exobiology”, SETIQuest, Volume 3, Number 1, pp. 8-16, First Quarter 1997 [Article]

(9) Sean N. Raymond, Rory Barnes and Noel Gorelick, “A Dynamical Perspective on Additional Planets in 55 Cancri”, The Astrophysical Journal, Vol. 689, No. 1, 478–491, December 2008