Debra Fischer: Details of the Centauri Hunt

by Paul Gilster on December 3, 2009

You won’t want to miss an interview with Debra Fischer now available on the MarketSaw site. The latter is a blog focused on 3D motion pictures, and thus the interest in Fischer’s work on Alpha Centauri draws from a cinematic base. Specifically, James Cameron’s new movie Avatar depicts a gas giant with a habitable moon around it, and the MarketSaw editors are interested in whether such a planet could exist around one of the Centauri stars. The interview that follows, discussing Fischer’s ongoing hunt for Centauri planets, is prime reading. I’ll quote from it, but you’ll want to read the whole thing (thanks to Vincenzo Liguori for the tip).

As to the gas giant question, we can answer that one quickly. Neither Centauri A nor Centauri B is orbited by a gas giant. We know this because enough data have accumulated on the question to rule such planets out. Stable orbits, says Fischer, don’t reach out much further than 2 AU around either star, and the lack of gas giants leaves smaller worlds a serious possibility, with the always tantalizing thought that there might be a terrestrial planet in the habitable zone. Fischer clarifies the orbital distances possible this way:

The stars themselves are in a very elongated, eccentric, elliptical orbit. At the widest part of their orbit, they are almost 40 AU (40 times the earth to sun distance) apart. That’s a comfortable separation for orbiting planets. But, when the stars, A and B, make their closest approach, they are only 11 AU away from each other. The stars are moving into the domain of the planets, and they will gravitationally rip away any planets that aren’t closer than 2 AU around “A” or “B”. Several computer simulations have demonstrated this. If you start the simulation with a system that has more distant planets, as the other star comes in closer, the planets are gravitationally disrupted, dislodged from their stable orbits. The orbits first become eccentric, and then the planet shoots right out of the system.


So we’re talking about planets, if they exist, that are inside the orbit of the main asteroid belt in our own Solar System (centered about 2.7 AU from the Sun). Fischer goes on to describe the work using a 1.5-meter telescope in Chile, as well as the Keck and Lick Observatory instruments. Why has a dedicated Alpha Centauri search taken so long? Fischer notes that the game has changed. Early on we were looking at lots of stars to build up a catalog of planets. Now we’re ramping up the precision to try to find Earth analogues. The ‘high cadence’ observations Fischer is making began in earnest on January 1 of 2009 and study Centauri A and B intensively:

We use one telescope, but we nod back and forth. The exposure times are 15 seconds for A, and 30 seconds for B. So we take ten observations of A, and then we move over to B and take ten observations there, then nod back to A and so on. So it’s just one telescope going back and forth. This is helpful because if we see the velocities of both stars just jump up or down for some reason, this will be a clue that there’s some kind of systematic error, and I hope that we’ll be able to track it down.

Image: The skies over the Cerro Tololo Inter-American Observatory, where Fischer’s work continues. Credit: San Francisco State University.

As to why Centauri B seems the more likely candidate for a planetary detection, Fischer has this to say:

As stars go, alpha Cen A and B are inactive. But, we don’t know if there are velocity fields at the sub-meter-per-second level. The stars are not completely smooth cue balls in space, like pool balls on your table. They have atmospheres, and those atmospheres have flares. In addition, the stars themselves pulsate. We know that Alpha Centauri “A” has a dominant pulsation period that’s about 5 minutes, just like the sun. I’m not really worried about periodic variations of minutes. We’ll be able to average right over that kind of noise. But no one knows whether or not there are long period variations in “A” or “B”. We do know that “A” is probably not as stable as “B”. People have measured pulsation periods in “A”, and in general we find that more massive stars have more active atmospheres. So yeah, I agree that “B” is a better bet. And if you told me that I could only observe one star, I’d choose B. But we’re studying both stars. And it turns out that this strategy of looking at both stars is pretty critical in ensuring a solid set of data.


Any signal that turns up will only be what Fischer calls a ‘tiny whisper,’ but a confirmed planetary signature might well boost interest in future space missions like SIM Lite or a direct imaging Terrestrial Planet Finder mission. Interestingly, the Centauri stars seem well situated for the current tricky work from the ground, with an orbital plane of 79 degrees — any planets are assumed to inherit the same orbital tilt. The reflex velocity of the stars is thus more or less along the line of sight from Earth. Were the system perpendicular to us, the radial velocity technique would be ineffective.

Image: Planet hunter Debra Fischer. Credit: Wikipedia/Creative Commons.

Fischer’s instruments, and those of Michel Mayor’s team at La Silla, are sensitive enough to detect a Centauri planet down to Mars size, depending on the stability of the stellar atmospheres. We may well find one or more small, rocky worlds, at which point the question becomes whether or not such a planet could have oceans — we don’t know what close binaries may do to water delivery from asteroids and comets. Whatever the case, though, a Centauri planet in the habitable zone would be a potent stimulus to the imagination and an obvious target for interstellar probes of the future.


Kenneth December 3, 2009 at 12:35


If they locate something, anything, of interest what should the next step program look like to get a complete understanding of the Alpha Centauri System. SIM Lite does a little as would TPF, but if we began to concentrate on the Alpha Centauri system as just an extension of our own Solar System and began seeking the same level of detailed data as we would for our own Solar System what should this comprehensive program look like. Obviously, Webster Cash would be involved, but what else would it take to do a Comprehensive Survey out to 5-12 Light years from our Sol/Terra System. That is one Comprehensive Survey or Situational Awareness Program we should be defining now, Lets have one interdisciplinary International program of Space and Terrestrial measurement techniques that is focused out to 12 Light Years to provided equistite detail of what is out there.


Erik Sørnes December 3, 2009 at 13:39

This is great. I’m really looking forward to the results. Maybe the planets have highly eliptical orbits as well, since the dual-star-system have elilptical orbits.
Did the models tell anything about this?


Carl December 3, 2009 at 16:05

“I’ll bet my career that there are planets around Alpha Centauri A AND B.” -Greg Laughlin, professor at UC Santa Cruz. Great quote from the Fischer interview. In time, I’m sure all the local stars will be scrutinized for planets. It’s marvelous that the science of exoplanet detection and characterization has come this far in this comparatively young field. Should the Centauri system lack major bodies, it could still harbor minor and dwarf planets.

It is still uncertain whether Proxima is gravitationally bound to the binary; at a trillion miles separation, a discovery of an Oort-type cloud around the three stars could yield clues about that from their orbits.

Enzo December 3, 2009 at 16:42

After one year of observation they must know something, one way or the other. I went through the interview hoping that she would give something away but she didn’t . No hints there, except maybe that they have not abandoned observations. Unfortunately that’s not enough of a hint because a real scientist would finish the job even if preliminary results are negative.

Ronald December 3, 2009 at 20:44

“We do know that “A” is probably not as stable as “B”. (…) in general we find that more massive stars have more active atmospheres. So yeah, I agree that “B” is a better bet. And if you told me that I could only observe one star, I’d choose B.”

This confirms what I just said in the previous thread on Alph Cen: it seems that A is already getting to bright and hot for comfort in its (former) HZ and I would definitely put my money on B.

John Hunt December 4, 2009 at 1:51

This is really critical research. Although we already know of a planet around Epsilon Eridani (10 light years away), there’s no great push to launch an interstellar probe there. But 4.2 light years would be so much closer. And just the fact that the Alpha Centauri system is our (second) nearest star, finding a rocky planet there would beg for us to come for a visit!

But I ask, if such a planet is found around one of the Alpha Centuari stars, will we be able to take advantage of the interest and present a credible proposal for an interstellar mission to reach it? If we have no consensus on what that mission should look like and we have no significant engineering plans and/or our rationale could not justify the expense, then the moment of opportunity will pass and the public’s attention will move on to other things.

The Constellation Program tantalizingly proposed the “Moon, Mars, and beyond”. A true interstellar mission is the logical ultimate fulfillment of that “beyond”.

philw1776 December 4, 2009 at 9:48

Reality check time. Way, way too early to consider sending a probe to the Centauri stars. Look at the length of the Pluto mission, thousands of times closer. After years of development, we’re still years away from flying a simple test version of VASIMR, a huge improvement in potential spacecraft velocity if it really works, but inadequate to send a mission taking less than many thousands of years. Stars are really far away.

Far better to push the technology to put large planetary ‘imaging’ systems in space. Scopes capable of spectroscopy of any planets within a few tens of LYs. Hard enough to get the several billions for such a much easier mission. Remeember, the decades discussed holy grail of TPF is still not funded and only consists of paper studies. There are NO space based planetary search missions in the que after Kepler unless you count the JWST whose real mission statement is far IR. These space missions don’t get congresscritters re-elected like TARP vote buying programs. Space expenditures will be first up in the Kabuki Theater of faux cuts to reduce spending.

kurt9 December 4, 2009 at 13:32

If Kepler is successful, I think we should lobby congress to fund the Terrestrial Planet Finder, even if this requires postponement of other scientific missions (as well as the end of NASA’s manned space program). I think the TPF represents the most effective of all of the proposed missions to find and characterize Earth-like planets in our stellar neighborhood. TPF will be rather expensive for an unmanned, scientific mission. But I think for the purposes of exoplanet science, as well as for space science in general, it would be worth the expense. The TPF is the logical successor to Kepler and WISE.

andy December 4, 2009 at 16:24

Regarding habitability of A, the interesting question is how the luminosity has evolved since zero-age main sequence (ZAMS). Playing around with the Yonsei-Yale isochrones interpolator, it appears that Alpha Centauri A has likely increased its luminosity by a similar fraction to the Sun, by virtue of the fact it is more metal rich thus has lower luminosity and longer lifespan than a solar metallicity star of the same age and mass.

Ronald December 5, 2009 at 7:32

@kurt9: I agree, focus should be on TPF and Darwin like missions, maybe an international joined effort is possible.

And I still wonder whether, for such relatively ‘modest’ distances, even a ground based ELT (30 meter diameter of more) on the Antarctic Plateau might do for direct imaging and spectroscopy. Recent research has revealed that that is by far the clearest and most suitable place on earth. Obviously a ground based observatory is less risky and much more maintenance friendly.

philw1776 December 5, 2009 at 11:32

“And I still wonder whether, for such relatively ‘modest’ distances, even a ground based ELT (30 meter diameter of more) on the Antarctic Plateau might do for direct imaging and spectroscopy. Recent research has revealed that that is by far the clearest and most suitable place on earth.”

Agreed. Although I’d want an environmental impact study considering the risk of penquin dropings done first.
Puting all our big dollar eggs into the JWST, which will be inaccessable to repair seems very risky.

kurt9 December 5, 2009 at 14:06


I agree. Darwin is the other mission that should be pursued.

andy December 5, 2009 at 15:03

Although I’d want an environmental impact study considering the risk of penquin dropings done first.

Birds are known to be inimical to physical experiments. They leave “white dielectric material” on the radio telescopes, they drop baguettes into the particle accelerators…

Theo December 5, 2009 at 17:15

Always a pleasure to read our wonderful blog, Paul. Like a lot of people I’ve often wondered why there wasn’t more research on Alpha Centauri, given its two suns that are so similar to ours. Now, it seems, the pace is picking up. If they ever do find a habitable planet there, no probe would ever get there in our lifetimes, I think, but it would be wonderful all the same to know if there were oxygen and water on such a planet.

Administrator December 5, 2009 at 22:39

Theo, you’re so right. If we were to find a planet in the habitable zone around Centauri B, what a stimulus it would be to further exoplanet research, and to the idea of someday sending a probe to such a relatively near star. Like you, I’m delighted to see the pace of Alpha Centauri research picking up.

kurt9 December 7, 2009 at 2:19

If rocky planets are spotted in Alpha Centauri system, it will help provide more support for the financing of TPF and Darwin missions.

Ronald December 7, 2009 at 9:34

@kurt9: maybe an open door, but if any rocky planets were discovered in the AC system, particularly in the HZ, it would be an enormous boost for any and all exoplanet research. And if a biosignature were detected, the sky would be the limit.
A new kind of ‘Apollo era’.

kurt9 December 7, 2009 at 13:17


True. Once we start getting results in from Kepler, probably late next year, I plan to write my congresscritters urging their support for TPF and Darwin (I would like to see both of these funded). They are not particularly interested in space, but I know what to say to spark their interest in this.

I am not a great fan of government-funded space projects, but this exoplanet research is certainly worth pursuing.

Tobias Holbrook December 8, 2009 at 5:49

Say we find a planet. Say it has life on it. Say that life has the potential to develop intelligence. What then? Who ‘owns’ the system? The inhabitants of the hypothetical planet, or any alien (i.e. us) colonists who move in? Sci-Fi literature appears to have reached a consensus on the ownership of planets – the natives own it, and if there are no natives, it can be claimed by anyone – but I still think we’d be pretty peeved off if a starship decelerated into the solar system, packed full of colonists who then went on to set up shop on Luna, Mercury, Venus, Mars, Ceres, Jupiter etc, even if they kept off Terra. It’s our system, dammit, and they have no right to be here…

Of course, that isn’t an issue if two postulates are true – 1) that we are the only intelligent species around, and 2) that we don’t engage in any uplifting activities on the native life. However, while I think 1 is true, I’m not so sure about number 2.

To use 2010: A Space Oddyssy as an example – the humans weren’t allowed to land on Europa. But they were given all the other worlds around it, despite it being the Europans home system.

Aaargh! I’ll write a paper on it, citing people like John Locke, shall I? Then try getting it published?

Ronald December 8, 2009 at 9:30

@Tobias Holbrook: the same could even be argued up to galaxy level. Now that I think about this issue, the level of ‘non-intervention’ largely depends on the civilization level: planetary, solar system, …, galaxy, …, cluster, …, supercluster (‘go away, it’s *our* supercluster’).

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