Barnard’s Star: No Sign of Planets

by Paul Gilster on August 16, 2012

Barnard’s Star has always gotten its share of attention, and deservedly so. It was in 1916 that this M-class dwarf in Ophiuchus was measured by the American astronomer Edward Emerson Barnard, who found its proper motion to be the largest of any star relative to the Sun. That meant the star soon to be named for him was close to us, and unless we’re surprised by a hitherto unobserved brown dwarf, Barnard’s Star remains the closest star to our Sun after the Alpha Centauri triple system. Stick around long enough and Barnard’s Star will close to within 3.75 light years, but even if you make it to 10,000 AD or so, the star will still be too faint to be a naked eye object.

Image: Barnard’s Star, with proper motion demonstrated, part of an ongoing project to track the star. This image shows motion between 2004 and 2008. Credit: Paul Mortfield & Stefano Cancelli/The Backyard Astronomer.

Peter van de Kamp, working at Swarthmore College, had been looking for wobbles in the position of Barnard’s Star going all the way back to 1938, and for a time his results indicated at least one Jupiter-class planet there and possibly two. But other astronomers failed to find evidence for planets, and later work raised the likelihood that the changes in the star fields van de Kamp was looking at were caused by issues related to the refractor he was using. Now we have a new paper from Jieun Choi (UC Berkeley), whose team went to work on Doppler monitoring of Barnard’s Star and concluded that van de Kamp’s findings were erroneous.

The paper, however, gives a generous nod to van de Kamp’s work:

The two planets claimed by Peter van de Kamp are extremely unlikely by these 25 years of precise RVs. We frankly pursued this quarter-century program of precise RVs for Barnard’s Star with the goal of examining anew the existence of these historic planets. Indeed, Peter van de Kamp remains one of the most respected astrometrists of all time for his observational care, persistence, and ingenuity. But there can be little doubt now that van de Kamp’s two putative planets do not exist.

The one-planet model fails to fit as well when studied with radial velocity data from both Lick and Keck:

Even van de Kamp’s model of a single-planet having 1.6 MJup orbiting at 4.4 AU (van de Kamp 1963) can be securely ruled out. The RVs from the Lick and Keck Observatories that impose limits on the stellar reflex velocity of only a few meters per second simply leave no possibility of Jupiter-mass planets within 5 AU, save for unlikely face-on orbits.

The paper goes on to drill down to planets of roughly Earth mass, finding no evidence for such worlds. The result is interesting on a number of levels. We’re finding smaller planets with radii 2 to 4 times that of Earth around M-dwarfs regularly in data from the Kepler mission, in an area close in to the star where this new study of Barnard’s Star is most sensitive to Earth-mass planets. The transit data back up radial velocity data on M-dwarfs from the HARPS spectrograph, which have shown numerous planets with mass a few times larger than Earth’s around M-dwarfs. A 2011 study found the occurrence of super-Earths in the habitable zone is in the area of 41 percent for M-dwarfs, leading to what Jieun Choi and colleagues describe as ‘a lovely moment in science.’

After all, our two major planet-hunting techniques — Doppler measurements to detect planets by their effect on the host star, and brightness measurements for transit detection — both indicate that small planets are apparently common around M-dwarfs. By contrast:

…the non-detection of planets above a few Earth masses around Barnard’s Star remains remarkable as the detection limits here are as tight or tighter than was possible for the Kepler and HARPS surveys. The lack of planetary companions around Barnard’s Star is interesting because of its low metallicity. This non-detection of nearly Earth-mass planets around Barnard’s Star is surely unfortunate, as its distance of only 1.8 parsecs would render any Earth-size planets valuable targets for imaging and spectroscopy, as well as compelling destinations for robotic probes by the end of the century.

Let’s not forget that the early work of Peter van de Kamp had energized speculation about missions to Barnard’s Star. The British Interplanetary Society’s Project Daedalus chose it as a destination largely because of its supposed planetary system even though Alpha Centauri was considerably closer (4.3 light years vs. 6). Robert Forward toyed with Barnard’s Star in his fiction, writing Flight of the Dragonfly, later expanded as Rocheworld, to depict both the planetary system there as well as the technology needed to reach it.

Now we have 248 precise Doppler measurements of Barnard’s Star from the Lick and Keck Observatories saying that the habitable zone of this conveniently nearby star appears to be empty of planets of roughly Earth mass or larger. Let’s hope the Alpha Centauri stars yield a better result. The paper is Choi et al., “Precise Doppler Monitoring of Barnard’s Star,” available online.

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{ 24 comments }

Tom Mazanec August 16, 2012 at 10:37

I am afraid the two stars, in an elliptical orbit at the prime planet forming distance, has ejected any planets in the Alpha Centauri A/B system. Perhaps Proxima Centauri offers a better chance for studying extrasolar planets close up.

Ronald August 16, 2012 at 11:22

Well, Barnard’s star is a (very) old population II star, and these are also generally very metal-poor stars. The most metal-poor among these are the galactic halo stars. Barnard’s star is not even a halo star, but an intermediate population II star, with between about 10 and 30% of solar metallicity.

Although recent research indicates that metallicity has smaller (negative) implications for the occurrence of small planets than for the occurrence of giant planets, there seems to be a lower limit (around 30% or so), below which any planets will be very scarce or absent.

In this respect it is not so surprising that old halo and even intermediate Population II stars show a scarcity or absence of planets in general.

From this we learn where to focus our (future) search efforts. ‘No’ is also an answer.

Mike August 16, 2012 at 12:22

Ofcourse this doesn’t rule out planets at Barnard’s Star. There could be lower mass planets in wider orbits that have so far eluded doppler detection. They could be in face on orbits. Darn, never have a SIM when you want one.
I think they’re there. I think it’s likely almost all main sequence stars have planets with very few rare exceptions. Drill baby drill.

Erik Sørnes August 16, 2012 at 15:59

Interesting, but wouldn’t a planetary system aproximately face on make room for larger planets in th hz within the statistical probabilities of the servey’s ? Anybody has any idea of the rotational axis of Bernards star ? And also, planets a bit smaller than earth could probably also exist. Something between earth and mars, perhaps is not improbable.

Michael the Civilized August 16, 2012 at 16:01

Isn’t this situation now considered quite unusual?

Paul Gilster August 16, 2012 at 17:12

Tom Mazanec writes:

I am afraid the two stars, in an elliptical orbit at the prime planet forming distance, has ejected any planets in the Alpha Centauri A/B system. Perhaps Proxima Centauri offers a better chance for studying extrasolar planets close up.

Paul Wiegert and Matt Holman showed in 1997 that stable orbits can exist within 3 AU of Alpha Centauri A or B. A major issue is whether planets could have formed there in the first place. More recent work is sprinkled through the archives here — this may be of interest:

http://www.centauri-dreams.org/?p=15048

and here’s another piece about prospects around Centauri B:

http://www.centauri-dreams.org/?p=11083

Proxima is intensively studied with no planets yet but the possibility indeed remains. A recent article on Proxima planets:

http://www.centauri-dreams.org/?p=22651

Enzo August 16, 2012 at 17:32

Regarding Alpha Centauri, look at fig.5 (at the very end) in this paper from Laughlin and others :

http://www.oklo.org/wp-content/images/AlphaCenApJPaper.pdf

It looks to me that, if there are planets around it, the various teams observing should already know. Maybe not enough to announce yet, but the signal should be noticeable considering that they started observing in 2008.
Maybe that explains Marcy’s call for a probe to be launched a while ago.

James D. Stilwell August 16, 2012 at 23:47

Wish we could detect binary planetary systems….Wouldn’t it be interesting to discover an earth or Mars sized moon with water and atmosphere and continents and so forth….dreamer that I am….or have we all been born too soon? Protect yourself….JDS

James Jason Wentworth August 17, 2012 at 2:59

Main sequence stars that have *no* planets may, ironically, be becoming more interesting because -they- are the oddities (the dynamical differences during their formation that left them “planet-less” would be interesting to study); what a change from just forty years ago, when we thought our solar system was the oddity, eh? :-)

andy August 17, 2012 at 3:05

@Ronald: on the other hand, thanks to Kepler we do know of a planetary system around a star very similar to Barnard’s Star. Kepler-42 (a.k.a. KOI-961) has three planets with radii somewhat smaller than Earth. Judging by the radial velocity limits and assuming reasonable values for the densities of the Kepler-42 planets they are not quite able to exclude such a system around Barnard’s Star even if it is close to edge-on.

Joy August 17, 2012 at 5:21

Possibly Barnard’s Star was ejected from a triple star system or a cluster, in which case it would not be expected to take any planets with it.

Mike Lockmoore August 17, 2012 at 12:32

Is it just me or does the animated image (Barnard_Mortfield_Cancelli_small_labels.gif) included in the text show a little lateral wobble? I mean, it seams to trace a subtle ‘S’ curve as it moves along compared to the background stars. Is this an artifact from slightly misregistered frames in the animation, or could it be due to anual parallax of Earth’s motion? I know the RV is too small, but the animation suggests a whopping big perpendicular velocity! :-)

Holger August 17, 2012 at 14:10

@Enzo: I think the ongoing searches have performed far less than the ~60,000 (97,000*3/5) measurements required (as I understand it) for detection in that Figure 5…

Eric August 17, 2012 at 14:25

The tone of this blog posting seems unduly pessimistic. Lets quote the paper’s conclusion:

“For orbital periods under 10 days, planets with M sin i greater than two Earth masses would have been detected, but were not seen. For orbital periods under 100 days, planets with minimum masses under ∼ 3 M⊕ would have been detected, but none was found. For periods under 2 years, planets with minimum masses over 10 M⊕ are similarly ruled out. ”

If I understand the above correctly, Venus, Earth, Mars, and additionally, a planet 1.5 times the mass of the Earth, all would not have been detected by this search, regardless of how closely they orbited Barnard’s star.

David August 18, 2012 at 2:33

Mike Lockmore,

Well spotted, yes the combination of parallax due to the Earth’s motion around the Sun, and the star’s own proper motion will produce that effect. Barnard’s Star also has the largest proper motion known.

Enzo August 18, 2012 at 7:25

@Holger : maybe, but if that’s the case, it is not what was planned.
In this interview Debra Fisher states that some observations started in Aug 2008, in Jan 2009 “the project started in earnest”.
http://marketsaw.blogspot.com.au/2009/10/eyes-on-alpha-centauri-hunt-for-pandora.html
She says “That’s when the program ends – in 4 or 5 years we’ll be done”.
Search for that phrase in the article.
So far, the only partial result for is for B “No planet larger than 4 Earth mass with a period shorter than 300 days”. I believe it to be an HARPS result.

Holger August 19, 2012 at 10:19

@Enzo: Ah, was this paper supposed to be a blueprint of Fischer’s own search? Then you may be right with your conclusion. (Although the figure only referred to a 1.7 Earth mass planet; actual Earth-sized planets would presumably take longer than 3 years.) I don’t actually know how many observations Fischer has already made, I was guessing Fischer’s number based on HARPS having made less than 1,000 as far as I know.)

The strange thing is that Fischer recently requested “privately funded” further telescope time via the Planetary Society, without disclosing anything about her search results of now 3-4 years’ time. (She was supposed to give a talk about her search in May 2011, but it was cancelled and not yet done, as far as I know.)

Eniac August 19, 2012 at 20:35

Eric is right. The data presented would not rule out a single one of the planets we have in our own system. So, cries of woe about lack of planets around Barnard’s Star surely are premature.

Ronald August 20, 2012 at 6:05

@andy: yes, point taken, but, taking Eric’s and Eniac’s comments about detectability into due consideration, it is also well possible that Barnard’s Star’s metallicity is even lower than that of Keper-42: only 10-30%, versus about 33%, and that this metallicity may be below an absolute lower threshold for planets to form. Time will tell.
I could not find information whether Kepler-42 is also an old halo or intermediate Population II star, or a younger one.

Enzo August 20, 2012 at 7:01

@Holger
“Ah, was this paper supposed to be a blueprint of Fischer’s own search? ”

I don’t know, I just noticed that she said that it would be all over in 4-5 years.
It might very well be that hings didn’t go to plan and that they got less measurements that they expected.

Holger August 20, 2012 at 8:54

I agree that these are not so bad news. The habitable zone of Barnard’s star corresponds to orbital periods of ~6-24 days, so we could still have a planet there with up to 3 Earth masses. (That limit is probably the lowest known for any (non-pulsar) star’s planets, but still high enough.)

@Eniac: Technically you’re right, but I’d expect smaller stars to have their planets closer to them, and the habitable zone is closer to Barnard’s star by a factor of ~20, compared to the Sun. When “shrinking” the Solar System’s orbits by such a factor, Jupiter and Saturn analogues can be ruled out around Barnard’s star.

spaceman August 20, 2012 at 14:19

I cannot say I am too surprised by this result. A low-metallicity red dwarf star, such as Barnard’s star, is more likely, according to the literature, to host Mercury to Earth-sized worlds than it is likely to host Earth to Neptune-sized worlds just based on the amount of rocks and ices that were likely there in the first place billions of years ago.

So, these are important limits on the types of planets that can exist around Barnard’s star– meaning that close in super-Earths are very likely not there; however, what about Earth and sub-Earth-sized worlds?

Bounty August 20, 2012 at 18:36

I wonder how difficult it is to account for parallax when looking at RV in such close stars.

Alex Ross November 11, 2012 at 9:44

This hardly excludes any planets around Barnard’s Star. It excludes the bigger ones. I think that at least a couple of sub-Earth size worlds exist there (not necessarily in the habitable zone, but definitely close to it.

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