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Franck Marchis

Proxima Centauri b: Have we just found Earth’s cousin right on our doorstep?

Posted by Franck Marchis

24-08-2016 12:01 CDT

Topics: extrasolar planets, explaining science

This post also appears at Franck Marchis' Cosmic Diary blog and is cross-posted here with his permission.

What began as a tantalizing rumor has just become an astonishing fact. Today a group of thirty-one scientists, led by Guillem Anglada-Escude at the Queen Mary University of London, UK, announced the discovery of a terrestrial exoplanet orbiting Proxima Centauri. The discovery of this planet, Proxima Centauri b, is a huge breakthrough not just for astronomers but for all of us. Here’s why.

Artist's impression of the surface of planet Proxima Centauri b

ESO / M. Kornmesser

Artist's impression of the surface of planet Proxima Centauri b
This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.

Astronomers know that exoplanets, or planets in orbit around stars other than our own, are numerous in our galaxy. NASA’s Kepler Space Telescope, which spent four years staring almost continuously at a tiny, distant patch of the sky, proved that there are on average two exoplanets per star in the Milky Way. That’s a lot of worlds! Kepler has also shown that most of our galaxy’s planets are indeed terrestrial—that is, like Earth or even bigger (so-called “super-Earths”). Unfortunately, the exoplanets discovered by Kepler are very distant—too far away for us to observe (and thereby study) using techniques available to most scientists today.

But Kepler is not our only tool for finding exoplanets. We can also use the radial-velocity method, which involves looking for the wobble a planet induces in its host star. This is an effective way to determine the mass of the planet and is not used only for distant stars. In the year 2000, for example, two astronomers announced the discovery of Epsilon Eridani b, a Jupiter-like exoplanet that we now call Aegir, orbiting around Epsilon Eridani, a bright star that’s only 10 light-years away. It was, until today, the closest exoplanet known. Now we know of one that’s far closer.

We now know of 3,374 exoplanets, an enormously large number, given that we discovered the first one only in 1995. Like the cartographers of the seventeenth century, who slowly build a map of our world, astronomers are drawing a map of our galactic neighborhood. We think we have a good handle on the location of nearby stars—that is, ones that are less than 50 light-years away. We know their distance, size, temperature, and if they are multiple systems or single stars, for example; but ultimately what we would really like to add to this 3D map of the galaxy are the planets in orbit around these stars.

The Pale Red Dot group was particularly interested in finding planets around Proxima Centauri, the star closest to the Sun. Proxima Centauri is only 4.25 light-years away, so it’s in our cosmic backyard. Because of its small mass, it’s too faint to be seen with the naked eye, and was discovered only in 1915. At the end of the 1990s, astronomers tried to detect potential large planets in orbit around this star using the radial-velocity method and came back empty-handed.

In the article published today in Nature, a group of modern astronomers reported on what they learned by using two high-precision radial-velocity instruments: HARPS at the 3.6m telescope of La Silla and UVES at the VLT 8m class telescope, both part of the European Southern Observatory. Several of these observations were done as part of other programs that took place between 2000 and 2016, but from January 2016 to March 2016, the team collected what we call high-cadence data, a fancy way to state that the star was observed once per night to increase its chance of detecting a tiny variation in its motion (about a meter per second, or the speed of a human walking) that might be caused by the presence of a small planet.

Artist's impression of planet Proxima Centauri b

ESO / M. Kornmesser

Artist's impression of planet Proxima Centauri b
This artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image between the planet and Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.

This ambitious program has paid off beyond our wildest dreams in that we have now unambiguously detected a planet with a minimum mass 1.3 times that of Earth orbiting the star right in the middle of the goldilocks zone (0.05 AU). I am not a specialist in radial-velocity measurement, but this detection seems quite convincing in that it has a false-alarm probability of less than 0.1% and uses a careful comparison of star activity (done by using additional small telescopes during the survey) that are known to mimic the signal of a planet. That is a very significant new data point to add in our cosmic map.

Did we find a terrestrial planet?

We don’t know for sure. The planet’s MINIMUM mass is 1.3 Earths because we don’t really know the orientation of the orbital plane with respect to the observer. (The radial-velocity method provides a measurement of m sin i, with i being the inclination of the system with respect to us.) Assuming random orientations of orbital planes, we have a 90% probability that the true mass is less than 2.3 times the minimum mass, so 3 Earths. In short, this could be a super-Earth or something more exotic, like a baby-Neptune.

Have we found a cousin of Earth?

Not yet. We don’t know the composition of the planet—keep in mind that we haven’t seen it but only its effect on its star. Consequently, we don’t have much information on the planet itself, but we do have a constraint on its mass (see above) and its orbit (one year of Proxima Centauri b is 11.2 days). Since the planet does not transit its star, we don't know yet its size, hence its density.

Can life exist on this planet?

The planet is at the right place to have a temperature that allows the presence of liquid water on its surface. The question of habitability is however very complex. We need to confirm that this is a terrestrial planet. The best way to do that would be to directly image the planet using the giant telescopes equipped with extreme adaptive optics that are currently being built (i.e., the E-ELT, TMT, GMT). The angular separation between the star and the planet is 39 milli-arcsec, so a telescope as large as 30 m could resolve the system with the right instrument, detecting the planet and possibly giving us insights into its composition.

Can life thrive on this planet?

This planetary system is different from ours. Proxima Centauri is a M-type star that is known for sporadic flares, or outbursts of energy. Those luminous UV and X-ray flares could have sterilized the surface of the planet regularly and/or ejected a significant part of its atmosphere into space. The authors briefly discussed the possibility of habitability given the possible presence of an extreme environment. I am betting that several follow-up papers on this topic will be published very soon. Astrobiology has taught us that life on Earth is resilient and can be found in extreme environments like deep oceans or protected from UV light in underground caves, so the possibility of a life somewhere in Proxima Centauri b cannot be rejected.

Ultimately, this discovery is a significant step on the road to mapping our galaxy. And it has given us a new world to explore, and one that is not too far away. We may not go there any time soon, but it will motivate us (and our funding agencies!) to design and build instruments to image and characterize this planet. What could possibly be more exciting than, in the not-too-distant future, getting a picture of a terrestrial planet whose atmosphere we can see and on which we could possibly detect signatures of life? That monumental moment may come in the next decade, and will definitely happen faster now that we know where to point our telescopes.

Let me close by saying that it is astonishing that we were able to detect this small planet after only three months of observations. The Pale Red Dot group is planning similar campaigns for other nearby stars. In the future, for example, we may know if Alpha Centauri A and B, another nearby system composed of two stars almost identical to the sun, host a true cousin of the planet we call home.

We have no way of knowing where this quest will go, or when, or what it will find. But clearly, this could be the most astonishing journey in the history of humanity.

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Or read more blog entries about: extrasolar planets, explaining science

Comments:

PeterD: 08/24/2016 04:32 CDT

Great summary, even if it turns out to be a mini Neptune it is a fantastic target for observation/exploration

Stephen: 08/25/2016 06:06 CDT

This is great news! Hopefully there will be more studies of it in due course that will narrow down its mass tell us more about it. However, I seem to remember reading somewhere that planets orbiting in the goldilocks zone of red dwarfs were likely to be tidally locked. If so, then quite apart from the flare issue the article mentions, that would also not be good.

David E: 08/26/2016 05:06 CDT

I think the latest claims should be treated with some caution. A claimed detection of a planet around Alpha Centauri B was recently withdrawn. In particular, it is worth noting that previous radial velocity surveys of Proxima Centauri found a completely different periodicity of 83.5 days. This was attributed to the rotation of the star and not to any exoplanet. In the latest survey, a periodicity of 11.2 days was found, but this was attributed to an exoplanet instead. This I find very curious. The new measurements appear to contradict the finding of a rotational period of the star of 83.5 days. Surely a stellar rotational component should show up in the radial velocity? If the rotational period of Proxima is NOT 83.5 days, then what is it? The obvious answer, I'm afraid, is that the new (and we assume improved – due to much better instrumentation) radial velocity periodicity is down to the rotation of the star, and that period is 11.2 days. All stars rotate, but not all stars have planets. Elementary reasoning (Occam's Razor) requires us therefore to assume that stellar rotation is the most parsimonious explanation for periodic radial velocity variations. We can only impute the existence of an exoplanet once the rotational effect has been otherwise accounted for which, in the present case, does NOT appear to have been done. Again, NO EVIDENCE of the 83.5 day rotation period has been found in the latest data. It is also, I would suggest, suspicious that 11.2 days is a very typical rotation period for a star (the Sun rotates in about 25 days at the equator).

Michael Richmond: 08/26/2016 01:00 CDT

@David E. wrote > NO EVIDENCE of the 83.5 day rotation period has > been found in the latest data. If you read the Nature paper which describes the latest result, http://www.eso.org/public/archives/releases/sciencepapers/eso1629/eso1629a.pdf you will see on page 4 that the authors discuss a variation of about 80 days in the brightness of the star, which they note is very close to the earlier 83-day period found in earlier studies.

David E: 08/26/2016 01:38 CDT

Hi Michael, the problem is that the 83.5 day rotation period came from a radial velocity observation campaign that has now been superceded by the new data. Check out http://adsabs.harvard.edu/abs/1998AJ....116..429B. Since the new analysis shows no radial velocity periodicity at this timescale, we cannot assume that this figure for the rotation period of Proxima is correct. This means that there is no way to correct for the rotation period of the star from the current data because it is no longer known. As I said, the 11.2 day signal could, in fact, be the actual correct rotation period. The use of radial velocity measurements to detect exoplanets, especially around red dwarf stars like Proxima, is fraught for precisely this reason. See http://arxiv.org/pdf/1604.03143.pdf. What is scientifically doubtful in the paper is using data (presumed rotation period of Proxima) to validate a result which has been specifically refuted BY YOUR OWN RESEARCH!

LocalFluff: 08/27/2016 02:37 CDT

Is Gaia capable of confirming Proxima b? First data release in September, finally a decent map of our neighborhood.

Tim: 08/27/2016 11:16 CDT

@PeterD, I'd also agree that Franck Marchis has provided an excellent summary of the situation. @Stephen, given the proximity of the planet to its host star, there will be a strong gravitational influence so complete tidal locking or Mercury-style 3:2 rotation–orbit resonance locking are strong possibilities. While there might be stronger UV, X-ray and solar wind fluxes, etc. if the planet is terrestrial in nature then it might have a reasonably strong protective magnetic field given its mass and moderate rotation rate (assuming it has a conductive fluid inside the planet). Again, if it is a terrestrial world, then it's possible that it's a volcanically active planet with a still-hot core given its minimum mass of 1.3 x that of Earth, i.e. it is less likely to be a dormant world like Mars. That has further implications because if there's active vulcanism then there's planetary outgassing so an atmosphere of one sort or another is likely.

Tim: 08/27/2016 11:18 CDT

Part II ike I said, there's lots of "ifs" there and this world is certainly worthy of much greater study. Coincidentally, I think it's provided a wonderful boost to the Starshot project. Finally, I don't think we shouldn't forget the exoplanet in the habitable zone around the star Wolf 1061 (14 light years away). That's surely an indication that humanity ought to be looking in greater depth at the nearest stars within a radius of two dozen light years or so for more habitable zone planets.

Torbjörn Larsson: 08/27/2016 07:45 CDT

@David: "Since the new analysis shows no radial velocity periodicity at this timescale, we cannot assume that this figure for the rotation period of Proxima is correct." I believe the paper claims differently. Besides four photometric time series recovering a 80 nights modulation, they report that spectroscopic activity - which would be radial velocity dependent, I believe - is consistent with that. [ http://www.eso.org/public/archives/releases/sciencepapers/eso1629/eso1629a.pdf ; pp 4-5.]

David E: 08/28/2016 01:17 CDT

The paper mentions the 11.2 day signal and that "A second signal in the range of 60 to 500 days was also detected, but its nature is still unclear due to stellar acti vity and inadequate sampling" So there's no proof whatsoever of an 80 day signal. If there WERE proof of an 80 day signal, the next question would be: is the 80 day signal actually an exoplanet and the 11.2 day signal Proxima's rotation?

Reed: 08/28/2016 03:09 CDT

@David E I see no indication that the most recent data invalidated the 83 day rotation period. On the contrary, Anglada-Escud´e et al clearly state the photometry they took at the same time as the RV data is consistent with the expected rotation period. The 1998 Fritz et al paper you linked was Hubble photometry, not RV. The photometry from both papers does not appear to be consistent with your suggestion that 11.3 days could be the rotation period. Such a short period would also be inconsistent with the inferred age of the star. You seem to be making an unfounded assumption that just because rotation can produce false positive in RV, it must always do so.

David E: 08/28/2016 07:05 CDT

Thank you Reed, your comments are highly illuminating. Exactly what this website is for!

Tim: 08/28/2016 07:46 CDT

@Torbjörn and @Reed, if this exoplanet does indeed have a longer rotation period such as 83 days then, all other relevant factors being present, that could indicate that any planetary magnetic field would be weaker than if it had a shorter rotation effect. As we can see from Venus in our own solar system, there's no significant dynamo effect and no significant generated planetary magnetic field because of that planet's sluggish rotation rate. I suspect the rotation rate matter will only be resolved by more studies over the next few years.

Reed: 08/28/2016 02:46 CDT

@Tim: ~83 days is the rotation rate of the *star*, not the planet. The planet is very likely to be tidally locked (rotation period = orbital period = ~11.3 days) although could possibly be in spin/orbit resonance like Mercury. The presence of a magnetic dynamo depends on many things. A paper released after the discovery discusses some models: http://arxiv.org/abs/1608.06919 One other comment on the original post: It says the planet does not transit, but in fact transits not yet been found or ruled out. Data from a recent observing campaign with MOST is still being analyzed by David Kipping's Cool Worlds lab.

LocalFluff: 08/28/2016 03:36 CDT

If I understand this correctly, the star's rotational period is determined by sunspots changing its brightness and redshift. Could it be that a planet so close to the star somehow interact (gravitationally, magnetically) with the star to cause a permanent star spot underneath it? Regardless of the star's rotation. A similar phenomena is observed on Jupiter, with the Jovian moons producing spots in its northern light. So that the presence of a planet would look like a stellar rotation.

Bob: 08/28/2016 07:49 CDT

The Planetary Society should start building grassroots support for a space probe to Proxima Centauri b for the next generation of planetary scientists. Perhaps a good reason for solar sailing. would be wonderful if a mission could be launched during my lifetime.

Tim: 08/29/2016 10:25 CDT

@Reed, thank you very much for the clarification for that is appreciated and things make much more sense now. @Bob, I am sure that the discovery of this particular exoplanet has provided a significant boost for the Starshot project not least because they now have a definite and nearby planetary target to aim at.

Stephen: 08/30/2016 06:25 CDT

@Bob: Any probe to Proxima Centauri this century is surely out of the question, especially if you want it to do more than a quick fly-through. The technology is simply not available, nor likely to be available any time soon. And I am not just talking about propulsion technology. Given the communications time-lag, the probe would have to be entirely independent of Earth assistance. Not only will it have to be able work out when to do any course corrections, if something goes wrong 2 or 3 ly out it cannot go into some special mode and wait for Earth to figure out what the problem is and upload a solution. it will have to be able to do so itself. The same go for the encounters at Proxima. In all likelihood much of the planning of what observations to make and when will have to be made by the probe itself. At best those back on Earth will only be able to tell the probe the kind they would like it to make and the respective priorities of those. The kind of artificial intelligence technology which can handle all that is still decades away. There is also the issue of how the probe will power itself, especially in interstellar space and particularly if the journey takes more than a few decades. However, even at Proxima itself that question is going to arise. Proxima is a VERY dim star so solar cells are probably out of the question, but so is plutonium, NASA’s current solution for outer solar system missions. Pu-238’s half-life isn’t nearly long enough for such a mission. Some other solution will need to be found. All of which suggests a better objective for the Planetary Society would be for it to build grassroots support for: a) research into the technology required for such a probe (because only when that technology becomes available will such a probe become technologically possible); b) NASA’s proposed interstellar probe, which won’t go to Proxima or any other star but might be seen as a necessary precursor for such a Proxima probe.

eltodesukane: 09/06/2016 08:19 CDT

"We must go to the stars. To another planetary system Where earth-like worlds have been detected. To the system of Alpha Centauri." quote from movie: The Time Travelers (Depths of the Unknown, 1964) near 24:40

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