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https://www.universetoday.com/165858/a-star-passed-through-the-oort-cloud-less-than-500000-years-ago-it-wasnt-the-only-one/

Stars travel throughout the Galaxy. It's inevitable that some will pass
near the Sun and perhaps even through our Oort Cloud, with interesting
consequences. Credit: NASA/ESA/STScI.
POSTED ONFEBRUARY 23, 2024 BY CAROLYN COLLINS PETERSEN

A Star Passed Through the Oort Cloud Less Than 500,000 Years Ago. It
Wasn’t the Only One.
As stars in the Milky Way move through space, some of them have an
unexpected effect on the Solar System. Over time, one comes closer to
the Sun during its orbit in the galaxy. Some of them actually get within
a light-year of our star and pass through the Oort Cloud. Such close
flybys can affect the orbits of the outer planets and send cometary
nuclei on a long inward rush to the Sun.

Astronomer Igor Yu Potemine at the Université Paul Sabatier in France,
and his colleagues decided to look for likely “close-passing” stars and
so-called “Nemesis” stars. Their tool was the SIMBAD database, which
contains updated stellar parallaxes and proper motions from ESA’s Gaia
satellite. They found a number of possible candidates. These stars
drifted through the outer Oort Cloud and then went back out to
interstellar space. Their actions set off gravitational perturbations
responsible for cometary visits to the inner Solar System over the past
billions of years. It’s important to note that gravitational influences
from the giant planets, as well as something called the “Galactic tide”
can also perturb objects in the Oort Cloud. For purposes of his study,
Potemine restricted his search to nearby stars as candidates for Oort
Cloud disturbances.

Stars and the Oort Cloud Region
When we look at which stars could cause a comet swarm from the Oort
Cloud region, a couple of types of stellar candidates come to mind. The
first is what some researchers call a “Nemesis” star. That’s the name
for a still-theoretical companion star to the Sun. It’s thought to be a
dwarf star that occasionally (like every 25-30 million years) passes too
close to the Sun. That action sends a swarm of comets to the inner solar
system. Astronomers continue to look for candidates for this solar
Nemesis, although the search hasn’t identified “the one” as yet. They
also look for other stars that periodically get too close to the Solar
System and even pass through the inner regions of the Oort Cloud.

A comparison of the Solar System and its Oort Cloud. 70,000 years ago,
Scholz's Star and companion passed along the outer boundaries of our
Solar System (Credit: NASA, Michael Osadciw/University of Rochester)
A comparison of the Solar System and its Oort Cloud. 70,000 years ago,
Scholz’s Star and companion passed along the outer boundaries of our
Solar System (Credit: NASA, Michael Osadciw/University of Rochester)
The Oort Cloud/outer solar system region is a still-largely unknown
place. It’s not one monolithic cloud but several regions with
populations of icy cometary bodies. The outer edge of the region could
extend out 3.2 light-years away from the Sun. Inside the Oort Cloud is
the Kuiper Belt, which also contains cometary bodies and a population of
small worlds such as Pluto, Eris, Makemake, and others. There’s also a
sort of intermediate population of cometary objects thought to exist
between the Oort cloud and the Kuiper Belt, sometimes referred to as the
Hills Cloud. This region may be populated with many more cometary nuclei
than the actual Oort Cloud. So, there’s plenty of material “out there”
for passing stars to perturb, and it’s likely many have in the billions
of years that the Solar System has existed.

Typically, you can expect a star to pass through (or near) the Oort
Cloud every hundred thousand years. Very close flybys (like within
52,000 AU, happen more rarely—about every nine million years. So, it’s a
fairly regular occurrence in the long history of the Solar System. The
star’s motion sets off gravitational disturbances that eventually jostle
cometary nuclei out of their orbits in the cloud. These “long period”
comets (named because of their extraordinarily lengthy orbits)
eventually pass by the Sun and then head back out to the depths of the
outer Solar System. The ones with the lengthiest orbits have only been
recorded once or twice in human history. That’s because some orbits can
be thousands of years long.

Lists of Likely Flyby Candidates
Patomine came up with several lists of likely “transgressor stars” from
the SIMBAD search. Some are the so-called “Nemesis” objects and others
are stars that have come closest to the Sun (within a light-year).
Further research needs to be done to establish precise orbits and proper
motions for all of the candidates. But, it’s interesting to look at a
few of them in more detail.

70,000 years ago, Scholz's star, a red dwarf, came as close as 1
light-year to our Solar System. It could have perturbed the Oort Cloud.
At that time, Neanderthals were still around. Image: Credit: José A.
Peñas/SINC
Scholz’s star, a red dwarf, once came as close as 1 light-year to our
Solar System. At that time, neanderthals were still around. Image:
Credit: José A. Peñas/SINC
One of the Nemesis candidates is Scholz’s star. It’s a red dwarf, and
likely grazed the edge of the Oort Cloud some 70,000 years ago, along
with a companion brown dwarf. Currently, it’s about 22 light-years from
the Sun and probably stirred up a swarm of comets that won’t get to the
inner solar system for more than a million years. It’s also likely that
its passage affected the orbits of Kuiper Belt objects. Others have also
been studied, including the G-type star HD 7977. It’s currently about
247 light-years from Earth and made its close flyby some 2.8 million
years ago.

Of course, there aren’t just past encounters to calculate. Other stars
will come near in the future. One of the best-known examples of a close
future passage is the star Gliese 710. It will fly past the Sun in about
1.29 million years at a distance of around 10,520 AU. It has a very good
chance of passing through the Oort Cloud, which means it could very well
perturb the Oort Cloud. That would send showers of comets toward the Sun
for millions of years. Some researchers estimate it could result in
about 10 naked-eye comets per year. Far from being a search for comets
of the past, the hunt for stars passing close to the Sun could also
predict an interesting future for observers on Earth thousands or
millions of years from now.

For More Information
Stellar flybies within 1 ly from the Sun and stars passing through the
Hills cloud
Gliese 710 Will Pass the Sun Even Closer

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CATEGORIESCOMETS, OORT CLOUD, SOLAR SYSTEM
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"a425couple" wrote in message news:oEQCN.567018$p%Mb.494@fx15.iad...

Stars travel throughout the Galaxy. It's inevitable that some will pass
near the Sun and perhaps even through our Oort Cloud, with interesting
consequences.
---------------------------

https://en.wikipedia.org/wiki/Pliocene

Until about 2.5 million years ago Earth's climate was several degrees warmer
than now, and fairly stable. Northern Greenland was forested. At that point
something started cyclic ice ages which appear to correspond to periodic
variations in Earth's orbit.

https://climate.nasa.gov/news/2948/milankovitch-orbital-cycles-and-their-role-in-earths-climate/

How we determine ancient temperature:
https://en.wikipedia.org/wiki/Oxygen_isotope_ratio_cycle

On 26/02/2024 00:08, a425couple wrote:
> It looks to me that for many of us, the newsgroups
> still work!
>

Why wouldn't they?

Apart from providing an easy way to search (until they broke even that),
google-groups was nothing more than a web front-end to usenet proper.