The uncertainty of science: Using computer models and the somewhat sparse data about the distant Oort cloud on the outer fringes of our solar system (from 2,000 to 200,000 astronomical units [au] away) and combining that with the data from the Gaia space telescope that mapped precisely the motions and distances of billions of Milky Way stars, astronomers now posit that the close pass of another star about 2.5 million years ago perturbed the Oort cloud and thus produced the shower of comets that humanity has been experiencing for the last few thousand years.

HD 7977 is a still nearby Sun-like star in the constellation Cassiopeia whose close passage was discovered by the Gaia mission. Approximately 2.5 million years ago, the orbits of the Sun and HD 7977 brought the two stars close together, but exactly how close is still an open question. Gaia data suggest they passed within 4000-25000 astronomical units of one another. Now, Kaib and Raymond have shown that the orbits of long-period comets suggest HD 7977 came within 6000-10000 AU of our Sun, setting off a major shower of new comets into the inner solar system.

You can read the preprint paper here [pdf].

These results are filled with many uncertainties of course. For one, the actual distance for HD 7977’s close pass is not well constrained. The margin of error is large, so that the star might have not done anything at all. Second, our map of the Oort cloud is very uncertain. In fact, it exists at this time only in theory, as it has never been directly observed. Astronomers hypothesize its existence based on the orbits of the long period comets that they have documented for the past few centuries, all coming from that distant region. So while it appears to exist, that existence remains unproven.

These uncertainties thus make the conclusions of this paper interesting but unconfirmed. Nonetheless, they are fascinating, because they are strongly suggestive, and hint at the impact of the galaxy and its stars on the evolution of our solar system itself. That impact is real, though tracing its history is difficult because of the vast time scales and distances. It appears the Gaia data and computers are giving us a first glimpse into that past history.