Mars, Venus — And A Long-lost Planet — May Have Once 'Waltzed' In Perfect Harmony...

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Astronomers have been increasingly interested in how planetary systems change their internal architecture on cosmic timescales, motivated by several recent exoplanet family discoveries, like the seven-planet cohort orbiting the tiny star TRAPPIST-1.

Past research has found that one early stage in a planetary family's metamorphosis involves pairs, triplets or entire systems moving in a rhythmic beat — called resonance — around their parent star.

Planets in resonance have orbital periods that form a whole-number ratio. In the TRAPPIST-1 system, for instance, the innermost planet, TRAPPIST-1 b, completes eight orbits for five of its nearest neighbor's.

Resonance arises among planets born within a protoplanetary disk — the disk of debris surrounding an infant star — that still contains gas.

Such planets plow through the gas, exchanging their rotational motion with it, which often causes them to move toward the star. Many of these planets may come close enough to each other for their orbital periods to "resonate," or become whole-number multiples.

But research from 2013 analyzing Martian isotopes suggested that terrestrial planets could have formed when the protoplanetary disk was still rich in gas, about 10 million years after the solar system's birth. This meant the terrestrial planets may have once been in resonance.

In all of the models, the team placed Saturn closer to Jupiter than it is today and had the rocky worlds grow by accumulating either pebbles or larger, trillion-ton rock blocks. In most simulations, Venus, Earth, Theia and Mars formed a 2:3:4:6 resonant chain within a million years of simulated time.