Centauri Dreams recently discussed the planets around HD 73526, as described in detail on astronomer Gregory Laughlin’s Systemic site. HD 73526c seemed attractive as a venue for life-bearing moons — a gas giant, it orbits well within its parent star’s habitable zone. The post inspired questions from readers on whether the chances for life on any large moons of such a planet would be minimized by Jupiter-style radiation fields. And given the unusual orbital resonance between the two planets, questions also arose about how these gas giants might have formed. Laughlin (University of California, Santa Cruz) was kind enough to answer these queries. His responses follow, with my inserted comments in italics.

The radiation environments around both HD 73526 b and c are probably more intense than in the vicinity of Jupiter. This increase would mainly be the result of the planets having larger masses than Jupiter, which gives them more vigorous interior convection and hence stronger magnetic fields. The planets also receive a larger flux of stellar wind particles as a result of orbiting closer to the parent star. The auroral displays on these planets are likely an awesome sight.

The fierce radiation environment also compromises the habitability of any moons in orbit around HD 73526 c. The moons would need to have strong magnetic fields of their own in order to stave off atmospheric erosion.

Image: HD 73526, a Sun-like star with a gas giant in the habitable zone. Credit: Space Telescope Science Institute (STScI).

Those ‘water worlds’ I speculated on yesterday in a comment on the original post may not be such comfy environments for life after all. And what about how these planets formed? Laughlin again:

Regarding the formation of the two gas giants orbiting HD 73526, I can
think of three (somewhat related) hypotheses. In decreasing order of
plausibility these are:

(1) The inner planet grew to its current large mass through the standard core-accretion process. It likely migrated inward to a distance of ~2AU after starting at the snowline of the protostellar disk (~5AU). The outer planet then formed and also began to migrate inward. When it reached a 2:1 orbital commensurability with the inner planet, the pair captured each other into resonance. Once in resonance, they both would have been forced to migrate inward together. As they migrated, their eccentricities were pumped up to the current fairly large values. This scenario does an excellent job of explaining the 2:1 resonant pair orbiting GJ 876. It is more problematic for HD 73526, however, because of the detailed dynamics of the observed resonance. In order for the scenario to work for HD 73526, the migration would have had to have been faster than expected, or the planets may have possessed an initial mutual inclination.

(2) The planets could have formed more-or-less where they are now, and captured each other into resonance as they became massive enough. I have seen core-accretion simulations which show that this scenario can work. The difficulty, however, is with growing the planetary cores fast enough in a region that is inside the protostellar snowline.

(3) The system could be the result of a catastrophic interaction between three planets whose orbits became dynamically unstable. In this scenario, one planet was ejected, and the other two were left in the observed, dynamically active, 2:1 resonant state. This sort of thing has been observed in simulations, but it is quite rare. In fact, if there had been a catastrophic event, one would expect the eccentricities of b and c to be even higher than observed.

For more on this unusual planetary system, see Laughlin’s recent post, and note that he has plugged the radial velocity data for HD 73526 into the Systemic Console for analysis. The console is a Java applet that allows users to manipulate such data and contribute to fine-tuning our tools for exoplanet detection. Laughlin’s Systemic research collaboration will ultimately work with a catalog of 100,000 stars, a fine case of applied distributed science.