Proxima Centauri

All posts tagged Proxima Centauri

Figure 1 (left) and 2 (right) from Anglada et al. (2017). The right figure shows a zoomed-in version of the left figure. The rainbow blob at the center of the left figure is Proxima Centauri’s debris disk, and the white ellipse shows the possible outer disk. The greenish blob just to the left of center in the right figure is the mysterious source, possibly a ringed planet.

In case you didn’t hear, late last year, astronomers confirmed a planet around our nearest stellar neighbor, Proxima Centauri, a red-dwarf star just four light years from Earth. The planet is probably about 30% more massive than Earth, probably making its composition Earth-like, and it’s in the habitable zone of its star, at a distance of about 0.05 astronomical units (AU) – all of which make it an exciting prospect for follow-up studies.

And just last week, Guillem Anglada and colleagues announced the further discovery of a debris disk around the star. The left figure up top shows the image, in radio wavelengths, of emission from the disk – the disk appears as the rainbow blob near the center, and the location of the host star Proxima is marked with a black cross.

The disk’s appears to orbit between 1 and 4 AU from its host star, which would put it between the Earth and Jupiter if it orbited in our solar system. However, since the red-dwarf star is so much smaller and cooler than our Sun, those orbital distances correspond to temperatures of only a few tens of degrees, making Proxima’s disk more akin to our Kuiper belt than our main asteroid belt.

The radio light we see from the disk is mostly due to thermal emission from dust. Using the above temperature estimate (and some other reasonable assumptions), Anglada and colleagues estimate (with large uncertainties) Proxima’s disk has about one thirtieth the mass of Ceres in dust and a lunar mass in larger bodies – almost as much mass as our Kuiper belt. There’s also marginal evidence in the data for a larger and cooler disk as well, perhaps 30 times farther from the star than the inner disk, and for something perhaps even more interesting.

In the right figure above, see the greenish blob just below and to left of the rainbow blob? That (admittedly weak) signal could be emission from a ring system orbiting a roughly Saturn-mass planet about 1.6 AU distant from the star. The authors point out that there’s a small but non-zero chance that it’s actually just a background galaxy that photobombed their observations, a possibility that can be easily tested by looking at Proxima again in a few months. But if it turns out to be a ringed planet, it would be the first exo-ring system directly imaged (other systems show possible signs of rings).

That would make Proxima an even more unusual planetary system since small stars tend to have small planets, and I’m only familiar with one other red dwarf star that hosts a big planet – NGTS-1 b, a red-dwarf hosting a hot Jupiter. But if there’s one thing that exoplanet astronomy has taught us in the last few decades, it’s to expect the unexpected.

The diagram below shows the structure of the Proxima Centauri system suggested by Anglada and colleagues.

Figure 4 from Anglada et al. (2017), showing the suggested structure of the Proxima Centauri planet-disk system.

A solar sail.

Very neat paper this week from René Heller and Michael Hippke exploring the exploration of the recently discovered planetary system around Proxima Centauri.

Proxima Centauri is the closest star to the Earth (only 4 lightyears away), and last year, an Earth-sized planet was found around it, opening to door to the very real possibility of a mission to an exoplanet.

After the planet’s discovery, the Breakthrough Starshot project proposed using solar sails and lasers to accelerate a tiny spacecraft to the system. Weighing only a few grams, the spacecraft could be accelerated to 20% the speed of light, giving a travel time to the system of about 20 years. Of course, the drawback to such a short trip is that the spacecraft would quickly zip past the planet, so the mission would have only seconds to collect data.

Building on that idea, Heller and Hippke pointed out that, as long as you didn’t mind waiting a little longer to get there (about 100 years), you could send the spacecraft at a low enough velocity that the solar sail could be used to slow the spacecraft on the other end. That would give you years to collect data, instead of seconds.

Key to their solution is the idea that you could slowly turn the solar sail, similar to tacking in the wind, to optimally slow and steer your 10-gram spacecraft. The animation below shows the basic idea.

With such a small spacecraft, there wouldn’t be a lot of room for moving parts to turn and orient the solar sail. To solve this problem, Heller and Hippke suggest the sail could be made of nanocrystals-in-glass whose reflective properties could be tuned to torque the spacecraft using the stellar photons themselves.

Of course, there are still zillions of technical problems to solve for such a mission (not to mention the difficulties of obtaining centuries-long NASA funding), but this study adds one more piece to the growing possibility of interstellar exploration.

Fig. 11 from Barnes et al. (2016) showing evolution of the HZ (blue region) of Proxima Centauri, along with the orbits of Proxima Centauri b (solid line) and Mercury (dashed line).

Fig. 11 from Barnes et al. (2016) showing evolution of the HZ (blue region) of Proxima Centauri, along with the orbits of Proxima Centauri b (solid line) and Mercury (dashed line).

As a follow-up to last week’s Proxima Centauri b event, we discussed a recent analysis of the planet’s habitability by Prof. Rory Barnes and colleagues in our weekly journal club.

In this paper, the authors consider a very wide range of evolutionary scenarios for Proxima b to explore the resulting range of outcomes and decide how habitable the planet is, really.

They incorporate lots of potentially important effects, including the evolution of the host star’s luminosity and its influence on the planet’s surface temperature.

M-dwarf stars, like Proxima Centauri, get dimmer early in their lifetimes. As a consequence, the surface temperature of a planet orbiting such a star can drop over time.

Or, put another way, the habitable zone (HZ) around the star can move inward, meaning planets that start out interior to the HZ (i.e., planets that might be too hot to be habitable) may eventually enter the HZ.

Figure 11 from Barnes et al. (2016) shows that this is probably what happened to Proxima b: it started out way too hot for habitability and, as its host star dimmed, it entered the HZ.

As Barnes et al. show, such a history could potentially drive away all the planet’s water (assuming it started with any), leaving behind a dried husk of a planet. But the fact that the planet is CURRENTLY in the HZ could fool us into thinking it’s habitable.

This result shows that planetary habitability is a complicated idea and that the current conditions on a planet can depend in a complex (and hard-to-determine) way on its history. Time (and lots more data) will tell whether Proxima b is actually an extraterrestrial oasis for life or a barren wasteland.

With the recent discovery of an Earth-like planet around the star Proxima Centauri, the nearest habitable world beyond our Solar System might be right on our doorstep. Celebrate this revolutionary find with Boise State’s Physics Dept on Friday, Sep 2 from 7:30p till 12a.

The event will kick off in the Multi-Purpose Classroom Building, Lecture Hall 101 (right across the street from the Brady Street Parking Garage) on Boise State’s campus with a public talk on the planet’s discovery from Prof. Brian Jackson.

Then at 8:30p the event will move to the Boise State quad (next to the Albertson Library and near the center of campus) the top of the Brady Street Garage (just off University Drive near Capitol), where telescopes will be set up to view Mars, Saturn, Uranus, and Neptune.

More information is available at or from Prof. Brian Jackson ( — 208-426-3723 — @decaelus).