"Long before it's in the papers"
February 18, 2015

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Little star probably grazed our solar system, astronomers conclude

Feb. 18, 2015
Courtesy of the University of Rochester
and World Science staff

As­tro­no­mers have con­clud­ed that 70,000 years ago, a ti­ny, dim star probably grazed the so­lar sys­tem—cross­ing its dis­tant cloud of comets, called the Oort Cloud. 

No oth­er star is known to have ev­er ap­proached our so­lar sys­tem this close—five times clos­er than the cur­rent clos­est star, Prox­i­ma Cen­tau­ri. On the oth­er hand, that’s still a thou­sand times fur­ther away from the Sun than the dwarf plan­et Plu­to.

Our own sun just looks like a very bright but ord­i­nary star (seen at left) in this artist's con­cep­tion of Scholz's star and its brown dwarf com­pan­ion dur­ing its fly­by of the so­lar sys­tem 70,000 years ago. (Cred­it: Mi­chael Os­ad­ci­w/U. of Roch­es­ter)


As­tro­no­mers reached the con­clu­sion af­ter an­a­lyz­ing the cur­rent speed and tra­jec­to­ry of a the now-fara­way star, nick­named “Scholz’s star.” The star al­so has a small com­pan­ion in the form of a brown dwarf, a type of “failed” star.
 
Re­search­ers in­clud­ing Er­ic Ma­ma­jek of the Uni­vers­ity of Roch­es­ter in New York re­ported the find­ings in a pa­per pub­lished in As­t­ro­phys­i­cal Jour­nal Let­ters.

The tra­jec­to­ry sug­gests, they said, that Scholz’s star passed roughly 8 tril­lion kilo­me­ters, or 5 tril­lion miles, away. This equals 52,000 Earth-Sun dis­tances away or about four-fifths of a light-year. 

A light-year is the dis­tance light trav­els in a year; our clos­est neigh­bor star Prox­i­ma Cen­tau­ri is 4.2 light years dis­tant.

The as­tro­no­mers say in the pa­per that they are 98 per­cent sure that Scholz’s star crossed the “out­er Oort Cloud” – a re­gion at the edge of the so­lar sys­tem filled with tril­lions of comets a mile or more across. Some of these have oc­ca­sion­ally been thrown off track and started or­bit­ing the Sun more close­ly, be­com­ing what are known as “long-period” comets, though Ma­ma­jek said Scholz’s star was probably not a ma­jor cause of this sort of event.

The star orig­i­nally caught Ma­ma­jek’s at­ten­tion dur­ing a dis­cus­sion with co-author Valentin D. Iva­nov, from the Eu­ro­pe­an South­ern Ob­serv­a­to­ry. Scholz’s star had an un­usu­al mix of char­ac­ter­is­tics in­clud­ing that it was ap­par­ently mov­ing di­rectly away from our So­lar sys­tem, and fast. Most stars as near­by as Scholz’s show more side­ways, or “tan­gen­tial,” mo­tion across the sky, said Ma­ma­jek. 

As­tro­no­mers es­ti­mat­ed its speed us­ing the Doppler-shift meth­od, which ex­ploits the change in light rays com­ing from a mov­ing ob­ject. This is anal­o­gous to what hap­pens when an am­bu­lance drives quickly away and the si­ren seems to drop in pitch. From this in­forma­t­ion, the re­search­ers in­ferred what the star’s past po­si­tion must have been.

Un­til now, the top can­di­date for the clos­est fly­by of a star to the so­lar sys­tem was the so-called “rogue star” HIP 85605, pre­dicted to come close to our so­lar sys­tem in the next quarter-to-half mil­lion years. But Ma­ma­jek and his col­la­bo­ra­tors said it won’t pen­e­trate the Oort Cloud, based on their new cal­cula­t­ions.

Ma­ma­jek car­ried out com­put­er sim­ula­t­ions of where Scholz’s star could have trav­eled, work­ing with form­er Uni­vers­ity of Roch­es­ter un­der­grad­u­ate Scott Baren­feld (now a grad­u­ate stu­dent at Cal­tech). Al­most all the sim­ula­t­ions showed the star pass­ing through the out­er Oort cloud. One out of 10 thou­sand run-throughs brought the star with­in the “in­ner” Oort cloud—which could have trig­gered a dis­as­trous “comet show­er” hit­ting Earth and oth­er plan­ets.

To­day, Scholz’s star is a small, dim “red dwarf” star in the con­stella­t­ion of Mo­noc­er­os, about 20 light years away. It would have been too dim to see even when it was “here,” but flare-ups due to mag­net­ic events could have changed that, once in a long while, for min­utes or hours at a time, Ma­ma­jek said.

The star weighs only an es­ti­mat­ed 8 per­cent as much as our sun, and its com­pan­ion about 6 per­cent. The for­mal de­signa­t­ion of the star is “WISE J072003.20-084651.2,” but it has been nick­named “Scholz’s star” to hon­or its dis­cov­er­er, as­tron­o­mer Ralf-Dieter Scholz of the Leibniz-Institut für As­t­ro­physik Pots­dam in Ger­ma­ny, who re­ported it in 2013.


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Astronomers have concluded that 70,000 years ago, a tiny, dim star probably grazed the solar system—crossing its distant cloud of comets, called the Oort Cloud. No other star is known to have ever approached our solar system this close—five times closer than the current closest star, Proxima Centauri. On the other hand, that’s still a thousand times further away from the Sun than the dwarf planet Pluto. Astronomers reached the conclusion after analyzing the current speed and trajectory of a the now-faraway star, nicknamed “Scholz’s star.” The star also has a small companion in the form of a brown dwarf, a type of “failed” star. Researchers including Eric Mamajek of the University of Rochester in New York reported the findings in a paper published in Astrophysical Journal Letters. The trajectory suggests, they said, that Scholz’s star passed roughly 8 trillion kilometers, or 5 trillion miles, away. This equals 52,000 Earth-Sun distances away or about four-fifths of a light-year. A light-year is the distance light travels in a year; our closest neighbor star Proxima Centauri is 4.2 light years distant. The astronomers say in the paper that they are 98% sure that Scholz’s star crossed the “outer Oort Cloud” – a region at the edge of the solar system filled with trillions of comets a mile or more across. Some of these have occasionally been thrown off track and started orbiting the Sun more closely, becoming what are known as “long-period” comets, though Mamajek said Scholz’s star was probably not a major cause of this sort of event. The star originally caught Mamajek’s attention during a discussion with co-author Valentin D. Ivanov, from the European Southern Observatory. Scholz’s star had an unusual mix of characteristics including that it was apparently moving directly away from our Solar system, and fast. Most stars as nearby as Scholz’s show more sideways, or “tangential,” motion across the sky, said Mamajek. Astronomers estimated its speed using the Doppler-shift method, which exploits the change in light rays coming from a moving object. This is analogous to what happens when an ambulance drives quickly away and the siren seems to drop in pitch. From this information, the researchers inferred what the star’s past position must have been. Until now, the top candidate for the closest flyby of a star to the solar system was the so-called “rogue star” HIP 85605, predicted to come close to our solar system in the next quarter-to-half million years. But Mamajek and his collaborators said it won’t penetrate the Oort Cloud, based on their new calculations. Mamajek carried out computer simulations of where Scholz’s star could have traveled, working with former University of Rochester undergraduate Scott Barenfeld (now a graduate student at Caltech). Almost all the simulations showed the star passing through the outer Oort cloud. One out of 10 thousand run-throughs brought the star within the “inner” Oort cloud—which could have triggered a disastrous “comet shower” with Earth among the targets. Today, Scholz’s star is a small, dim “red dwarf” star in the constellation of Monoceros, about 20 light years away. It would have been too dim to see even when it was “here,” but flare-ups due to magnetic events could have changed that, once in a long while, for minutes or hours at a time, Mamajek said. The star weighs only an estimated 8 percent as much as our sun, and its companion about 6 percent. The formal designation of the star is “WISE J072003.20-084651.2,” but it has been nicknamed “Scholz’s star” to honor its discoverer, astronomer Ralf-Dieter Scholz of the Leibniz-Institut für Astrophysik Potsdam in Germany, who reported it in 2013.