Science

7 interstellar objects of the Oumuamua type would visit us every year

The detection of the first two interstellar objects, in 2017 and then 2019, sparked a slew of scientific publications and literally launched a new field of research. The existence of such vagrants from and ejected by other star systems than ours, which according to their (very long) wanderings would break into the Solar System, had certainly been foreseen for a long time. The object which is finally reached us and which was discovered four years ago by the Pan-STARRS1 telescope in Hawaii, however, greatly surprised astronomers.

Oumuamua arrived from above the Solar System. Credit: ESA

A very fast and strange looking object

Because of its strange appearance, first of all, assimilated at first to a kind of “cigar” but which would actually look like … a “big pancake”! But above all of its kinematics – its movement in the vicinity of the Sun being both very fast and accelerated. Baptized 1I / `Oumuamua (” messenger “or” scout “in the Hawaiian language), the bolide of about 50 meters in diameter was thus considered, in turn, as a comet, an asteroid (most probable hypothesis), a hydrogen iceberg… and most recently like a fragment of an exoplanet saturated with nitrogen ice (like Pluto or Triton, one of Neptune’s natural satellites). Without forgetting the theory – at least improbable – of an extraterrestrial vessel, reaffirmed in a recent work by the American astrophysicist Abraham Loeb.

2017 and then 2019, sparked a slew of scientific publications and literally launched a new field of research. The existence of such vagrants from and ejected by other star systems than ours, which according to their (very long) wanderings would break into the Solar System, had certainly been foreseen for a long time. The object which is finally reached us and which was discovered four years ago by the Pan-STARRS1 telescope in Hawaii, however, greatly surprised astronomers.

Oumuamua arrived from above the Solar System. Credit: ESA

A very fast and strange looking object

Because of its strange appearance, first of all, assimilated at first to a kind of “cigar” but which would actually look like … a “big pancake”! But above all of its kinematics – its movement in the vicinity of the Sun being both very fast and accelerated. Baptized 1I / `Oumuamua (” messenger “or” scout “in the Hawaiian language), the bolide of about 50 meters in diameter was thus considered, in turn, as a comet, an asteroid (most probable hypothesis), a hydrogen iceberg… and most recently like a fragment of an exoplanet saturated with nitrogen ice (like Pluto or Triton, one of Neptune’s natural satellites). Without forgetting the theory – at least improbable – of an extraterrestrial vessel, reaffirmed in a recent work by the American astrophysicist Abraham Loeb.

Oumuamua was first considered to be a kind of “space cigar”, an oblong-shaped object. Credits: European Southern Observatory / M. Kornmesser

Hair produced by the sublimation of ice cream

Called 2I / Borisov (named after its Ukrainian discoverer, amateur astronomer Gennadi Borisov), the second interstellar object would be ten times larger: more than 500 meters wide obviously. And scientists this time had little doubt about its cometary nature, a “hair” produced by the sublimation of the water ices with which the star is filled having been observed in its wake. This interstellar comet is nonetheless the subject of active and very much research. At the end of March, two studies published in Nature Communications and Nature Astronomy have moreover brought astounding indications. They show that 2I / Borisov, whose optical properties are very different from most comets in the Solar System with the notable exception of Hale-Bop, is one of the most intact ever observed. Perhaps even the “purest” of all! Largely spared by stellar radiation, it would never have grazed the slightest star and thus very faithfully reflect the local composition of the extrasolar system which gave it birth.

Interstellar comet Borisov photographed on December 9, 2019 by Hubble. Credits: NASA, ESA AND D. JEWITT (UCLA)

A study based on 1.8 billion stars

Something to captivate scientists … and whet their appetites while waiting for new forays! Let them be reassured. According to the work of an international team of astronomers, currently being validated, these intruders cross the Solar System quite frequently. Based on the latest catalog from the European satellite Gaia, which covers more than 1.8 billion stars in the Milky Way, and establishing that a rocky body similar to 1I / `Oumuamua (say asteroidal) would roam in each 10 cubes 150 million kilometers per side of interstellar space, they calculated the speed and flow of such objects.

Even faster than Oumuamua

The result: seven such cars would cross the Solar System about one astronomical unit from the Sun, or 150 million kilometers, and “close” to Earth, in other words. 92% of them would come from the “thin disc” of the galaxy, which contains the vast majority of stars, against 6% of the “thick disc” located above the galactic plane and which mainly contains old stars. The rest would come from the “halo” that surrounds the Milky Way … and even rocky bodies (at a rate of 3 per century) from another galaxy than ours! Half of these objects would also have a speed greater than 40 kilometers per second. Faster than 1I / `Oumuamua therefore, which was already spinning at 26 kilometers per second when it approached our star before moving away from it never to return.

A new generation of telescopes

Interstellar comets like 2I / Borisov, on the other hand, are rarer: one every ten years, according to the authors of the study. So many predictions that will soon be tested by a new generation of extremely efficient telescope, such as the American Vera Rubin Observatory currently under construction in northern Chile and which will perhaps discover many interstellar objects! These estimates will also make it possible to better prepare various mission projects, such as the Lyra program (piloted in Great Britain) which is studying the possibility of one day flying over such visitors …

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