On March 11, 2013, the team operating the WISE infrared telescope made a surprising discovery. They found Luhman 16, a pair of brown dwarfs located 6.5 light years from our Sun. It is ranking as the third most distant star system known. Let’s quickly review this discovery, and unfold some of the valuable content.

What is the surface temperature of Luhman 16?
Luhman 16 system. Source: ESO / I. Crossfield / N. Risinger

The Nearest & Closing Stars of Earth

The question of which stars are closest to us may seem simple, but it took until the 20th century for humanity to get a reasonably accurate answer. For a long time, we knew little about the distances to other stars.

In the late 19th and early 20th centuries, it was a realization that most nearby stars are small and dim. They are rendering them practically invisible despite their proximity if we compare it to the well-known bright stars. These faint stars are red dwarfs. Barnard’s Star, Proxima Centauri, Wolf 359, and Lalande 21185 are examples of such stars, all closer to us than Sirius, which is 8.5 light years away.

The nearest neighbors of the solar system.
The nearest neighbors of the solar system. Source: Wikipedia

Luhman 16’s Brown Dwarfs

One of our closest ‘star systems’ isn’t made up of stars at all. Instead, it’s a pair of brown dwarfs, objects too small to shine like stars yet too big to be called planets. Interestingly, these brown dwarfs closely resemble Jupiter, our solar system’s largest planet.

The challenge lies in detecting objects even smaller and dimmer than red dwarfs, and that’s where brown dwarfs come in. They were predicted by theory but proved elusive to spot. Brown dwarfs tend to have lower surface temperatures, making their radiation peak in the red part of the spectrum, outside our visible range.

brown dwarfs
A pair of Luhman 16 brown dwarfs in images from the WISE telescope. Source: Wikipedia

Luhman 16, only six and a half light-years away and the third-closest system to the Sun, harbors these elusive brown dwarfs. It wasn’t until 2013 that they were finally discovered. Subsequent observations revealed that these brown dwarfs orbit each other every 27 years, with both being about Jupiter’s size but around 30 times more massive.

Luhman 16 System- The Third Solar System Near Sun

While a decade has passed since the discovery of the Luhman 16 system, much remains unknown. The larger of the two dwarfs, Luhman 16A, weighs in at 3.2% of the Sun’s mass, equivalent to 33 times Jupiter’s mass. Interestingly, it boasts a dense structure, with an estimated radius of only 85% that of Jupiter.

Luhman 16A, belonging to the spectral class L, maintains a surface temperature of 1,350 K (around 1,080°C). There’s a possibility it shines not from residual light but due to ongoing thermonuclear reactions inside.

Luhman 16B, on the other hand, with an estimated mass of 28.6 times that of Jupiter, presents challenges in determining its radius due to fluctuating brightness. It likely reaches surface temperatures of 1200 K and falls into the ‘medium’ class T.

Observations reveal that both brown dwarfs follow a slightly elongated orbit, with a semi-major axis of 3.5 AU and a 27.5 Earth-year period, akin to Saturn’s rotation around the Sun.

Spectroscopic studies uncover an abundance of alkali metals like potassium and sodium in brown dwarfs. However, their age remains uncertain, with estimates ranging from a minimum of 120 million years to a maximum of 3-4.5 billion years.

Does Luhman 16 have planets?

In the Luhman 16 system, close-in giant planets are notably absent. Extensive observations conducted with the Hubble Space Telescope between 2014 and 2016 definitively confirmed the absence of any extra brown dwarfs in the system. Furthermore, it dispelled the possibility of Neptune-mass objects, roughly 17 times the mass of Earth, with orbital periods ranging from one to two years.

What is the Luhman 16 binary system?

Luhman 16A and Luhman 16B. These brown dwarfs orbit each other and are located approximately 6.5 light years away from the Sun. Remarkably, Luhman 16 is the third-closest system to the Sun, following Alpha Centauri and Barnard’s star.

What is Luhman 16 made of?

One of our closest neighboring ‘star systems’ doesn’t actually include any stars. Instead, it consists of a duo of brown dwarfs. These objects aren’t massive enough to emit the brilliant light of real stars, yet they’re too substantial to be classified as planets.

What spectral class is Luhman 16?

Luhman 16A falls into the spectral class L, boasting a surface temperature of approximately 1,350 K (around 1,080°C). There’s a possibility that it doesn’t merely emit residual light but rather shines due to ongoing thermonuclear reactions within. As for Luhman 16B, it remains even more enigmatic.

How bright is Luhman 16?

Despite their faintness, Luhman 16 emits both heat and light. Although it’s far too dim to be visible to the naked eye from Earth. With a surface temperature of around a thousand degrees Celsius or 1350 Kelvin, it would produce a subtle red glow to our eyes.

What is the surface temperature of Luhman 16?

Luhman 16A, classified in the spectral class L. It has a surface temperature of 1,350 K (around 1,080°C), might not simply emit residual light. There’s a possibility that it shines due to ongoing thermonuclear reactions occurring within its core

The Constellation “Sails”:

Even though we can’t directly see them, it’s likely that molecules in Luhman 16 create cloud bands. These are similar to those seen on Jupiter. Some of these bands are thick, appearing dark, while others are thinner, making them brighter. Storms might also swirl through these bands and around the poles, reminiscent of Jupiter’s patterns. So, while Luhman 16’s brown dwarfs are unique in our solar system, they might have some familiar features.

Luhman 16 resides within the constellation Vela. It is ‘the sails.’ This constellation exclusively graces the southern horizon with its presence. It is in the evening hours from the southern US. Luhman 16 itself is quite faint and usually requires the assistance if you want to see.