Proxima b: The hunt for Proxima’s wobbler

This week a team led by Dr. Guillem Anglada-Escudé, an astronomer at Queen Mary University of London, reported in Nature the discovery of a potentially habitable planet orbiting the red dwarf star Proxima Centauri.

Quoting Dr. Anglada-Escudé:

“We know there are terrestrial planets around many stars, and we kind of expected the nearby stars would contain terrestrial planets. This is not exciting because of this. The excitement is because it is the nearest one.”

By “nearest one” Dr. Anglada-Escudé does not mean the closest exoplanet discovered to date. He means the closest exoplanet it is possible to discover. Proxima Centauri is our nearest star neighbor, just 4.25 light years from Earth.

Dr. Anglada-Escudé is a member of Pale Red Dot, a group of astronomers seeking to find habitable planets around nearby stars. The group’s name is a variation on Pale Blue Dot, the description of Earth made famous by Carl Sagan.

Beginning January 11, 2016 the group began measuring the “wobble” of the star Proxima Centauri, that is, the slight changes in the star’s position relative to Earth. These changes are shown as Doppler shifts in the star’s light as observed from Earth.

The inserted pic shows the red and blue shifts in the star’s light as it distance from Earth increases (red shift) or decreases (blue shift).

Stellar_Wobble

As shown the star’s position relative to Earth is changing due to the gravitational effects of an unseen planet. The dotted yellow line depicts the star’s orbit around the center of mass between it and its planet. This is known as a “wobble”.

Interestingly the wobble method of planet detection was predicted in 1952 by the noted astronomer Otto Struve. He foresaw that the development of sensitive spectrographic equipment would enable astronomers to prove that the star was being influenced by a planet. Unusually for his time, Struve was outspoken in his belief that planets were common throughout the universe and that life existed on some of them.

All wobbles are not as dramatic as the one in the pic. The relationship between the mass of a star and its planets determines the extent of the wobble.

Our Sun for example has a wobble. However, the Sun’s mass is so great relative to its planets that the center of mass is actually inside the Sun. There is no external point to orbit. As a result the Sun simply shifts in its place, measurably but not dramatically.

An example of a dramatic wobble is shown by the orbits of Pluto and Charon. These two bodies are so close in size that the center of mass for these two bodies actually lays outside both of them. In New Horizon’s imaging Pluto’s movement can be seen distinctly as Charon orbits it.

Certain characteristics of the planet can be inferred from the “wobble” it’s causing in its star. Distance from the star, composition, and the length of time it takes to orbit its star are among the inferences which can be made. While not precise these inferences have a high degree of probability mathematically.

From these inferences others can be made. Knowing the planet’s distance from its star, knowing the size of the star and knowing the amount of energy the star produces enables astronomers to estimate whether or not the planet is in the Goldilocks Zone, that is if it’s possible for the planet to sustain life as we know it.

The Pale Red Dot team used three observatories in the hunt for Proxima’s wobbler:

I. European Southern Observatory – HARPS:

ESO HARPS is located in La Scilla Chile approximately 370 miles north of Santiago.

HARPS is an acronym for High Accuracy Radial Velocity Planet Searcher. Radial velocity is the speed of a star along the line of sight of the observer.

HARPS is a spectrograph built specifically to look for exoplanets. Light from La Scilla’s 3.6 meter telescope is fed through fibers to HARPS which then calculates the radial velocity of the star being observed to an accuracy of 1 m/s, the highest level of accuracy presently available.

II. Las Cumbres Observatory (LCOGT):

LCOGT is a world-wide network of at least eighteen telescopes positioned to allow for continuous observation of a signal star.

LCOGT helped rule out that the possibility that changes in radial velocity were caused by Proxima Centauri’s solar flares.

III. ASH2 telescope located at San Pedro de Atacama:

San Pedro de Atacama is approximately 1,000 miles north of Santiago in Chile.

The telescope provided photometry which helped confirm the true radial velocity of Proxima Centauri.

Such an exciting discovery! A potentially habitable planet orbiting the closest star to Earth in the universe!

And some day we will go there.