PerspectivePlanetary Science

Forming Terrestrial Planets

Science  02 May 2014:
Vol. 344, Issue 6183, pp. 479-480
DOI: 10.1126/science.1252257

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The Sun's rocky planets come in a range of sizes. Mercury weighs in at barely 5% of Earth's mass, perhaps because it formed under harsh conditions close to the Sun. Venus is similar in size to our own planet. However, the modest mass of Mars is perplexing. Current theories for planet formation can explain the gross features of the solar system, such as the dichotomy between its rocky and gas-rich planets, but there is no consensus on why Mars is almost a tenth the mass of Earth and Venus. Now, simulations by Izidoro et al. (1) show that Mars' small size may date back to a partial gap in the solar nebula, the cloud of gas surrounding the young Sun.