As you may recall, we were in the process of taking our 20% speed-of-light spacecraft, perhaps based on the “Breakthrough Starshot” program, outward into nearby space to explore the star systems around us! It’s always a good thing to know your neighbors, and in the case of star systems, what potential for resources and chance for habitable planets they provide.

https://caucus99percent.com/content/science-escape-4-breakthrough-starshot

Initially, we stopped and looked at the first star system near us--Alpha Centauri. This was actually a very interesting star system with lots of potential.

https://caucus99percent.com/content/science-escape-5-hmmmm-where-go-wher...

Continuing our look outside our solar system for next possible star systems to visit, let’s start with the closest and continue outward from Earth.

Barnard’s Star (red dwarf--6.0 light years away)

The next closest star system to us is Barnard’s star, a very old red dwarf. Our 20% speed of light spacecraft would take about 30 years to reach this system (versus the 21-22 years for Alpha Centauri). If our galactic center was to the southeast of the Sun in the celestial sphere, Alpha Centauri be to the “south” of us and Barnard’s Star would then be about due east (and a little bit upwards in three dimensions).

The star is about 15%-20% of the Sun’s diameter.

Because of its nearness, Barnard’s Star has been the object of intense planet-finding interest. Decades ago, an astronomer thought he discovered two gas giant planets orbiting the star, but no subsequent investigations--with ground-based telescopes or spacecraft--have turned up any evidence of this. No wobbles, no regular changes in brightness . . . nothing.

Interestingly, even though a red dwarf, Barnard’s Star is not a “flare” star (see the previous Alpha Centauri link above for a discussion about flare stars). The dwarf is so old that it’s pretty calm, which would bode well for life on any planets that might be orbiting it. In 1998, there was one major flare that doubled the brightness and temperature of the star for a very short period of time, which was pretty dramatic. Still, as far as the stability of the star goes, Barnard’s Star fares better than most red dwarf stars.

Although searches for planets will continue, with no known planets and not much in the way of drama, Barnard’s Star seems to be a lonely, rather boring place. Let’s continue to look outwards.

But first, a primer on what we’ll find next . . .

Brown Dwarf Systems

Brown dwarfs were only proven to exist in 1994, although they had been theorized for some time before that. They aren’t really stars, and they’re not really planets. They are something in-between.

When solar systems form, basically a lot of dust, gas, and rock in an area starts to conglomerate via gravity, and the central point draws more and more mass into itself from the surrounding area. If the amount of mass amounts to about 80 times the mass of the planet Jupiter, the gravitational forces tend to crush everything together to such and extent that fusion of normal hydrogen starts to happen, and a star is born. The more mass, the larger the star, but 80 times the mass of Jupiter is the approximate cutoff where this fusion can start to take place.

Below 13 times the mass of Jupiter, a planet will form. There is not enough mass to form any kind of nuclear reaction. A large amount of mass will likely form a gas giant planet, a smaller amount of mass will form a rocky body, which can become a planet, dwarf planet, or asteroid, again depending on the amount of mass involved.

In between 13 and 80 Jupiter masses, fusion can start to take place, but only with deuterium molecules--a form of hydrogen that has only a proton and a neutron (typical hydrogen has a proton and an electron, with no neutrons). Deuterium is more susceptible to fusing, so the lesser mass is still enough to instigate it.

The result is not a full-fledged star, but more like a dimly glowing protostar, which is called a “brown dwarf.” The surface temperature is still as hot as a small red dwarf star (about 6,700 degrees Fahrenheit), but brown dwarfs (dwarves?) do not radiate much light because the amount of fusion is much smaller--there is relatively little deuterium compared to normal hydrogen. They also produce no radio waves or x-rays.

An important point to note is that brown dwarfs are only about the diameter of Jupiter, but they are much more massive (i.e., more dense).

Size of a brown dwarf compared to a red dwarf, Jupiter, and Earth. The Sun is not to scale in this illustration--it would be a lot larger. Public Domain image from NASA.

Luhman 16ab (dual brown-dwarf system--6.5 light years away)

In 2013, two brown dwarf stars were found orbiting each other, Luhman 16a and 16b, only 6.5 light years from the Sun. Luhman 16a has about 34 times the mass of Jupiter, and Luhman 16b has about 28 times the mass of Jupiter. This is a very young system, only about 600-800 million years old (compared to our 4.5 billion year old system). This system is in the general direction of Alpha Centauri (to the south if our galactic center is to the southeast).

Again, with our 20% speed-of-light spacecraft, it would take about 32-33 years to reach this system.

The European Very Large Observatory in 2014 managed to make an atmospheric heat map of Luhman 16b--the first ever for a brown dwarf (see image below). They attempted the same for Luhman 16a, but there was no heat difference across the dwarf in that case.

Image courtesy of ESO.

In 2013, wobbles in the orbits of the two stars were detected, suggesting a planet or third brown dwarf in the system. Studies by the European Very Large Observatory and Hubble Space Telescope failed to confirm any companions, so it may be that the companion is very far away from the pair, or smaller than Neptune.

Are habitable planets possible around brown dwarf stars? We still do not know very much about these types of systems, but with very little x-ray radiation and most emissions from these “stars” coming in the infrared range (i.e., heat), it is very possible. Perhaps Luhman 16ab would be an interesting place to visit with our spacecraft, just to learn more about how these systems work.

It’s kind of hard to imagine that we are still--just within the last few years--discovering nearby neighbors to our solar system!!!

In the next episode, we will continue onwards . . .

Sources:

http://solstation.com/stars/barnards.htm

https://en.wikipedia.org/wiki/Barnard's_Star

http://solstation.com/stars/wise1049.htm

https://en.wikipedia.org/wiki/Luhman_16

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