In our night sky, located in the constellation Ophiuchus, there is an unremarkable red dwarf
star some six light years away.
After the three stars of the Alpha Centauri system, this is the closest star to us.
Known as Barnard’s star, it is invisible to the unaided eye.
It’s name comes from it’s discoverer E.E. Barnard, who in 1916 noticed that this star
was wobbling back and forth by some 10.3 arcseconds every year.
Astronomers thought there must be some unseen companion orbiting around it, but for over
a century, couldn’t find it.
Was it a dim star?
Or was it a planet?
Why was this star wobbling?
Now, we know.
Astronomers using the planet-hunting instrument known as HARPS and others from around the
world, have pinpointed this unseen companion.
By measuring the spectrum of the star for long periods of time, astronomers measured
the shift in the spectral lines as the star wobbled.
When something orbits a star, its gravitational pull causes the star to move to and fro.
When the star moves away from the Earth, its spectrum redshifts; in other words, it moves
towards longer wavelengths.
Similarly, starlight is shifted towards shorter, bluer, wavelengths when the star moves towards
Earth.
These shifts can tell us a lot about the unseen companion around Barnard’s star.
The HARPS instrument can detect changes in the stars velocity as it wobbles as small
as 3.5 km/hr.
This method called the Radial Velocity method.
So, what’s in orbit around Barnard’s Star?
The observations from HARPS and seven other instruments spanning 20 years indicate that
there is a rocky, super-Earth sized planet circling the fourth closest star to us.
Called unimaginatively Barnard’s Star b, it has a mass at least 3.2 times that of the
Earth and it orbits its host star once every 233 days.
Because Barnard’s star is very small and dim, the light from Barnard’s Star provides
this planet with only two percent of the energy the Earth gets from the Sun.
Even though Barnard’s Star b is about twice as close to its star as the Earth is from
the Sun, the low energy the planet gets from the star means it is very cold, close to what
astronomers call the Snow Line.
If there is any water there it would be solid ice.
The temperature on this world is minus one hundred seventy degrees celsius, making life
as we know it here inhospitable.
The stars in our stellar neighborhood turn out to have many planets.
Alongside Proxima Centauri b, the seven worlds of TRAPPIST-1, Epsilon Eridani b and Ross
128, we can now add another: Barnard’s Star b, a cold super-Earth on the edge of habitability.