I’m late getting to this one, because I wanted to get Mike Gruntman’s paper on interstellar instrumentation finished. But for exoplanet enthusiasts like myself, the best news to come out of the recent American Astronomical Society meeting may have been the announcement of a new planet around the star HD 74156. So let’s talk about it, an interesting find because we haven’t had a new planet turn up just where predicted since Urbain Le Verrier and John C. Adams (independently) worked out the existence of Neptune by noting its effects on the motion of Uranus. Thus were calculations turned into observations and thence discovery.
Rory Barnes (University of Arizona) has been working on a theory that led to the HD 74156 discovery for some time. His computer simulations (begun with Thomas Quinn while both were at the University of Washington) on the stability of extrasolar planetary systems showed a key fact: All systems whose planets were close enough to affect each other gravitationally were on the edge of instability. All it would take would be a slight change in orbits to lead to disruptions. Astrophysicist Steven Soter explained the implications in an article last summer:
This remarkable result might seem surprising. But the prevalence of such marginally stable systems makes sense, Barnes and Quinn concluded, if planets form within unstable systems that become more stable by ejecting massive bodies. The investigators remarked, “As unsettling as it may be, it seems that a large fraction of planetary systems, including our own, lie dangerously close to instability.”
Mature planetary systems hold about as many planets as they can, spaced as closely as is consistent with stability. Think of an evolving planetary system as one that ejects disruptive elements before settling into its mature stage, a process of self-organization. If that hypothesis works, it becomes a useful predictor, as Barnes, Quinn and Sean Raymond (University of Colorado) saw. HD 74156’s two known planets were a test case, because the gap between them suggested the presence of a third. It took observations by Jacob Bean and team at the University of Texas to observe the system and make the actual discovery.
The question is, are the processes that proved so successful at HD 74156 likely to be universal? If so, we have a useful predictor of planets around other stars, helping us probe more deeply into systems we’ve already begun to study. Now comes word that another team has found a planet where the ‘packed planetary system’ theory suggested it would be around the star 55 Cancri. Thank you Debra Fischer and team.
55 Cancri f turns out to exist at the inner region of a large stable zone, suggesting the possibility of still further planets in this most interesting system. More on this from the 55 Cnc f discovery paper:
This ﬁfth planet apparently resides in the previously identiﬁed gap between 0.24-5.8 AU, and it remains between 0.73 AU (periastron) and 0.84 AU (apastron), preventing orbit crossings with both the next inner planet, “c”, whose apastron is at 0.26 AU and the outer planet, “d”, whose periastron is at 5.5 AU, ensuring dynamical stability that is demonstrated numerically by N-body simulations. As the star’s luminosity is L = 0.60 L⊙ (from its effective temperature and radius), this ﬁfth planet resides within the classical habitable zone. With a minimum mass of 45 MEarth, we speculate that it contains a substantial amount of hydrogen and helium, not unlike Saturn (M = 95 MEarth ) in the solar system.
The kind of calculations Barnes and team have employed also call for a planet in a stable region around HD 38529, although that one awaits confirmation. As we continue to explore the interesting hypothesis of ‘packed’ planetary systems, it begins to appear that efficiency is the watchword. If there is room for a planet to form without destabilizing gravitational effects, it does. And that tells us something about the ubiquity of celestial real estate.
The discovery paper is Bean et al., “Detection of a Third Planet in the HD 74156 System Using the Hobby-Eberly Telescope,” accepted by The Astrophysical Journal and available online. The 55 Cancri paper is Fischer et al., “Five Planets Orbiting 55 Cancri,” also accepted by The Astrophysical Journal and available here. Thanks to Dave Moore for helpful background links as this story developed, and to other Centauri Dreams readers who passed the story along and asked for comment.
Addendum: Be sure to check andy’s comment re a paper that may cast HD 74156 d into doubt.