October 22, 2021

The Sun was in a relationship with a star born at the same time as him

The Sun was born along with other sister stars in an open star cluster, now scattered throughout the Milky Way. It is believed that he might even have a twin brother (or twin sister, it depends) with whom he temporarily formed a binary star. This twin would explain the origin of the famous Oort cometary cloud.

 The researchers came to this conclusion based on statistics of pairs of young stars observed in nurseries such as the Perseus molecular cloud . The observations indeed suggested that all stars of masses comparable to that of the Sun were born in pairs before, sometimes, to separate. We also know that the majority of stars in the Milky Way are indeed in a relationship.

The idea continued to gain ground and it is today taken up by the famous Abraham Loeb (or Avi Loeb), customary of articles on arXiv with original ideas , even eccentric according to some. The researcher, director of the astronomy department at Harvard University, is however all that there is more serious. However, it seems that he has the habit of sometimes launching a young doctoral student by publishing with him a study with exotic and stimulating conclusions . The new theory published in  The Astrophysical Journal Letters does not seem to escape this scenario since its co-author, Amir Sira, is precisely not yet a doctor.

The Solar System was formed from a dust-rich molecular cloud collapsing under its own gravity. This is how the Sun was born, surrounded by a protoplanetary disk.

Loeb and Siraj therefore bring new arguments to the thesis that the Sun may have already had a similar mass binary companion and initially about 1,000 AU distant from our young star. To do this, they revisit a problem known for a while, that of the origin of the famous Oort cometary cloud. Let us recall some elements about it, and already exposed by Futura.

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An explanation for long-lived comets

In 1950, the Dutch astronomer Jan Hendrik Oort published the results of his work on  long-lived comets in an article which was the birth certificate of the discovery of what is now called in his honor the cloud of Oort . In this article, Oort relied on the fact that the orbits of long-period comets known in 1950 had their apheles at distances of the order of 20,000 to 100,000 astronomical units.(AU) of the Sun. He concluded that there existed, between these distances from the Sun, a set of comets rotating in roughly circular orbits. The inclinations of their orbits being arbitrary, this reservoir of comets must have a spherical symmetry.

One light year is approximately 60,000 AU. The nearest stars are just over four light years away, so 250,000 AU. This meant that some comets were traveling at a distance from the Sun on the order of magnitude  of the distance between the stars. A simple calculation showing that the orbital speed , at this distance, is only a few meters per second, while the speed stars close to the Sun is of the order of 20 km / s, it was concluded that the comets of the Oort cloud reservoir must be particularly sensitive to gravitational disturbances from other nearby stars. During its wanderings around the Milky Way, our Sun had to get close enough to other stars for comets of this cloud to change orbit to move towards the internal Solar System , under the effect of these disturbances.

Astronomer Jan Oort. © Jan Oort

A problematic genesis for the Oort cloud

From the formation models of the Solar System, it was deduced that these comets were remnants of the early stages of this formation and that they had in fact been ejected into long orbits by the influence of giant planets , in particular Jupiter . Each comet was therefore a fossil containing the primitive matter of the protoplanetary disk , thus put “in the fridge” for billions of years.

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But, as Futura explained a decade ago in an article from which all these considerations are extracted, a digital simulation conducted by Harold Levison and his colleagues led to a completely different picture of the origin of the Oort cloud.

Indeed, if the comets of the Oort cloud are indeed celestial bodies ejected into long-period orbits but having formed relatively close to the Sun, the simulations lead to estimate that the cloud could only contain a few billion of these objects. This estimate turns out to be very insufficient to account for the observations which lead to a number of several hundred billion such comets. On the other hand, if we take into account the fact that our Sun was formed with many other stars in an open cluster , by fragmentation of a cloud of dust and gas, then the disagreement between theory and observations can be eliminated. Thus, the Sun would have stolen comets from other planetary systems evolving in formation in parallel with it.

An extrasolar origin for planet 9?

This scenario is today reinforced by Loeb and Siraj who now argue that we can even more easily explain the filling of the Oort cloud by involving comet captures by a binary system made up of the Sun and its temporarily linked twin. to him by gravity before escaping his attraction and continuing his life somewhere in the Milky Way.

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So for Loeb ‘  are binary systems are much more efficient for capturing objects as single stars. If the Oort cloud forms as observed, this implies that the Sun in fact had a similar mass companion which was lost before the Sun left its birth cluster  ”. And the astrophysicist adds that “  the puzzle does not only concern the Oort cloud, but also extreme transneptunian objects, like the hypothetical Planet 9. We don’t know where they come from, and our new model predicts that there should be more objects with an orbital orientation similar to planet 9 ”.

The Vera C. Rubin observatory, formerly called LSST and which should see its first light in 2021, could confirm or not all these considerations by discovering in particular several dwarf planets . This opens up new perspectives for exobiology and the study of the formation of exoplanets , since there would be a way to directly sample primitive matter from other planetary systems. The origin of planet 9 itself, if it exists, could also be extrasolar . If these ideas were indeed a reflection of reality, who knows their implications for panspermia and the origins of life?