It's that time of year when cherry blossoms draw crowds across the world, but a few cherry trees this year have sprouted a mystery. Four cherry tree saplings across Japan blossomed this spring, as much as six years ahead of schedule. The trees all come from a selection of seeds that took a trip to the International Space Station in 2008 as part of an experiment. Over 250 seeds made the trip, and while many are being studied in labs, 14 were planted around Japan after spending eight months in space. This variety of tree typically takes no less than ten years to sprout, but a few had a big surprise in store this spring.

Scientists are now scratching their heads to try and figure out the premature blossoms. Kaori Tomita-Yokotani, a University of Tsukuba researcher involved in the experiment, tells the AFP that "there is the possibility that exposure to stronger cosmic rays accelerated the process of sprouting and overall growth." She adds that cross-pollination could also account for it, but scientists are struggling to find an answer because data on the seeds is currently very limited. On one tree, the mystery is only compounded by the flowers themselves: they only have five petals, compared with 30 on the parent tree. For now, there's no clear explanation. As Tomita-Yokotani says, "From a scientific point of view, we can only say we don't know why."

Correction: The cherry trees were originally expected to take ten years to blossom, which would roughly be during the year 2020 — not 2018 as originally stated.