Wednesday, July 16, 2014

Swinging on Highly Eccentric Orbits

A number of exoplanets are known to orbit their host stars in wildly eccentric orbits. These exoplanets have “comet-like” orbits where they come close to their host stars and then recede far out. One example is HD 80606 b - a Jupiter-like planet which orbits its host star in a highly elongated orbit with eccentricity 0.9336. The planet’s distance from its host star varies from 0.03 to 0.88 AU, where 1 AU is the average Earth-Sun distance. At closest approach, HD 80606 b receives over 800 times more insolation than when it is farthest from its host star. Near closest approach, temperatures on HD 80606 b can rise from 800 K to 1500 K in a mere 6 hours. HD 80606 b attains a maximum orbital velocity of 227 km/s when it is closest to its host star. At that speed, a sufficiently large meteoroid barrelling into HD 80606 b could produce a truly spectacular meteor on the planet’s nightside.

Figure 1: A model of HD 80606 b showing the stormy response of the planet’s atmosphere right after closest approach to its host star. Credit: NASA/JPL-Caltech/G. Laughlin et al.

In the Solar System, all known planets go around the Sun in relatively circular orbits. Nonetheless, besides comets, a number of asteroids are known to orbit the Sun in highly eccentric orbits. These asteroids have “comet-like” orbits that bring them from a far out locale, to a close swing around the Sun, and out again. One notable asteroid in this category is 2006 HY51 - a 1.2 km asteroid in an extremely eccentric orbit around the Sun. 2006 HY51 has a remarkable orbital eccentricity of 0.9688. It comes within 0.081 AU of the Sun (1/4 of Mercury’s closest distance to the Sun) and recedes as far as 5.118 AU from the Sun (grazing Jupiter’s orbit).

Figure 2: Artist’s impression of an asteroid.

2006 HY51 goes around the Sun ever 1530 days, spending the vast majority of its time further than Earth is from the Sun. It is believed to be an asteroid with a stony composition and not an inactive comet. When 2006 HY51 is closest to the Sun, it receives nearly 4000 times more insolation from the Sun compared to when it is at its farthest. The insolation it receives at closest approach is 150 times more intense than the insolation Earth gets from the Sun. From 2006 HY51, the Sun at closest approach would appear over 12 times larger than it would from Earth. The orbital velocity of 2006 HY51 reaches almost 150 km/s when it is swinging by the Sun at closest approach. In comparison, Earth orbits the Sun with an average orbital speed of 29.8 km/s. A few asteroids such as 2005 HC4 and 2008 FF5 have highly eccentric orbits similar to 2006 HY51 that take them very near the Sun. However, both are much smaller than 2006 HY51 and do not swing out as far.

- Moutou C. et al. (2009), “Photometric and spectroscopic detection of the primary transit of the 111-day-period planet HD 80606 b”, Astronomy and Astrophysics 498 (5): L5-L8
- Fossey S.J., Waldman I.P., and Kipping D.M. (2009), “Detection of a transit by the planetary companion of HD 80606”, Monthly Notices of the Royal Astronomical Society: Letters 396: L16-L20
- G. Laughlin et al., “Rapid heating of the atmosphere of an extrasolar planet”, Nature 457, 562-564 (29 January 2009)
- Yoonyoung Kim et al. (2014), “Physical Properties of Asteroids in Comet-like Orbits in Infrared Asteroid Survey Catalogs”, arXiv:1405.2989 [astro-ph.EP]