July 27, 2011 | 3
It’s an exciting time for solar system exploration. NASA’s Dawn mission has now entered into an orbit around the minor planet known as Vesta, a 500km diameter chunk of rock and who-knows-what-else that orbits the Sun beyond Mars and in the classical asteroid belt. After hanging out at Vesta for a year Dawn will pull up orbital anchor and strike out for the 1000km icy body known as Ceres, arriving in 2015.
Together these bodies represent the tip of the figurative iceberg in terms of small objects in our solar system, with at least 2 million chunks of rock and ice larger than 1km within the main asteroid belt, and vastly more smaller bits of detritus. They also represent key pieces of solar system history, quite possibly from the earliest stages of rocky planet formation, yet each with very different characteristics; Vesta is dry, Ceres is icy. Now it appears they may play an important and unexpected role in the gravitational evolution of our own local environment.
Coinciding with the Vesta encounter a new study of solar system dynamics by Laskar and colleagues reveals a rather remarkable and unexpected role for these minor chunks of planetary material. Despite their small mass – Ceres weighs in at about 1/10,000th times the mass of the Earth, Vesta at about a fifth that of Ceres – they apparently have a significant effect on the long-term orbital dynamics of the inner solar system, including that of the Earth.
We’ve known for a while that the orbital conditions of the major planets in the solar system are marginally chaotic. Exactly what past or future configurations of planetary orbits will be across 100 million years or so is therefore hard to pin down. The planet Mercury may fall into the Sun in a billion years or so, or even collide with Venus. Even the subtle effects of general relativity can profoundly influence such lengthy dynamical evolution. What hadn’t been fully explored before was the behavior of smaller bodies (minor planets and asteroids) and their influence on the larger planets.
Laskar et al. incorporated both Ceres and Vesta into their gravitational simulations, along with the other main asteroids Pallas, Iris, and Bamberga. Although they had attempted this before, this time they dug further into the details of the gravitational contribution of these bodies over very long time scales and found something quite remarkable. Not only are the orbits of the larger asteroids like Ceres and Vesta more chaotic than previously understood (limiting any predictability of orbital parameters to a window of less than 500,000 years – a veritable drop in the ocean of time), but their presence has a significant influence on the Earth’s orbital ellipticity (eccentricity). These tiny worlds perturb us. The upshot is that the Earth’s own orbital variations cannot be retraced or predicted for any length of time greater than about 60 million years.
In other words we cannot say what the elliptical shape of Earth’s orbit was more than 60 million years in the past. This shape, albeit typically only some 1.7% different to a circular orbit, can have profound influence on terrestrial climate. It exaggerates the seasonal variation of energy received from the Sun due to our planetary tilt, and that can drive significant climate change. So even small variations at the level of a few percent in the elliptical shape of our orbit can drive enormous change on the ground. As Laskar and his colleagues point out, this is a heck of a problem for paleoclimate studies. One might want to match the geological record of climate to the orbital history of the Earth, but the subtly nudging gravitational perturbations of Ceres and Vesta seem to be enough to make this impossible.
It’s also rather sobering news for the study of exoplanetary environments. We might hope to be able to label some terrestrial-type exoplanets as “habitable” on the basis of simple parameters such as current orbit, type of stellar parent, and planetary composition. But what our own solar system is telling us is that at best we can give these planets an AA rating, probably not an AAA rating – and that will be based on a single snapshot at this point in their orbital history.
The upside is that the more we learn about the idiosyncrasies of our own cosmic situation the better equipped we are to find new tools to probe both our own planetary history and that of new worlds.