Closing in on Alpha Centauri

by Paul Gilster on April 19, 2012

Alpha Centauri is irresistible, a bright beacon in the southern skies that captures the imagination because it is our closest interstellar target. If we learn there are no planets in the habitable zones around Centauri A and B, we then have to look further afield, where the next candidate is Barnard’s Star, at 5.9 light years. Centauri A and B are far enough at 4.3 light years — that next stretch adds a full 1.6 light years, and takes us to a red dwarf that may or may not have planets. Still further out are Tau Ceti (11.88 light years) with its problematic cometary cloud, and Epsilon Eridani (10.48 light years), a young system though one thought to have at least one planet.

A warm and cozy planet around the K-class Centauri B would be just the ticket, and the planet hunt continues. One thing we’ve learned in the past decade is that neither Centauri A or B is orbited by a gas giant — planets of this size should have shown up in the data by now. We’ve also learned that stable orbits reach out maybe 2 AU from either star. Remember that while Centauri A and B are separated by almost 40 AU at their widest point, they close to within 11 AU, thus disrupting outer orbits, as demonstrated by computer simulations. We should expect planets, if they exist, to be no further out than the main asteroid belt in our own system.

Helping the Centauri Planet Hunt

Debra Fischer (Yale University) has been working on the Alpha Centauri problem at Cerro Tololo (Chile) in addition to her efforts at improving instrument sensitivity for planet hunting at the Keck and Lick observatories. The goal is to reach the precision needed to turn up planets the size of the Earth with radial velocity methods. If we’re going to get a Centauri detection, odds are it favors Centauri B because A does not seem to be as stable as B, and the latter is more likely to be the first to yield what Fischer calls the ‘tiny whisper’ that would flag an Earth-like world. Usefully, the 79 degree orbital plane of these stars means that planets in this system, assuming they share this tilt, should be generating a reflex velocity close to the line of sight from the Earth.

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

Radial velocity methods, in other words, should work here if we can attain sufficient sensitivity. The detection effort calls for telescope time at the Cerro Tololo Inter-American Observatory this spring and summer, and The Planetary Society is campaigning to raise money to support the effort. What Fischer needs is 20 nights of observing time, but the team’s NASA and NSF grants cannot be used to pay for telescope time, which at Cerro Tololo runs to $1650 per night. A total of $33,000 will do it, then, money the community should be able to raise. Have a look at the Planetary Society’s donation page and let’s see if we can’t make this happen.

Anyone involved with The Planetary Society is probably already aware of Fischer’s work with astronomer and Tau Zero practitioner Geoff Marcy (UC-Berkeley) on FINDS Exo-Earths (Fiber-optic Improved Next generation Doppler Search for Exo-Earths). The collaboration has resulted in a high-end optical system installed on the 3-meter Lick Observatory telescope and is now feeding the FINDS 2 effort to provide advanced optics for the Keck Observatory in Hawaii. Marcy and Fischer are working with a fiber optics array that adjusts light entering the telescope’s spectrometer and an adaptive optics system that offers the best signal to noise ratio.

FINDS worked out well at the Lick Observatory, improving the ability to detect Doppler velocities from the pre-existing 5 meters per second down to the 1 meter per second range, allowing us to detect smaller planets. Fischer and Marcy are hopeful of attaining precisions down to 0.5 meters per second with their work at Keck, which should get us into the range of Earth-sized planets. FINDS 2 will then be used with Keck to provide follow-up data about planets found by the Kepler mission, ruling out false positives in the ongoing hunt for planets like our own. The work on FINDS has led directly into the commissioning of a new spectrometer at Cerro-Tololo.

The Cerro-Tololo instrument is now tuned up and perfectly positioned for the study of Centauri A and B. The system is a valuable target whether or not we find a habitable world (or even two) there. We don’t know whether a rocky world around Centauri A or B would have oceans, because the mechanisms for delivering water to inner planets in a binary system are unknown. What Alpha Centauri provides on our doorstep is a look at planet formation in a close binary system — recall that as many as half of all stars are in binary pairs, so these are questions with a profound relevance to our understanding of the galactic distribution of planets.

What we find around Alpha Centauri may also drive future space research. We’re gaining good information about the statistical spread of planets in the galaxy through the Kepler mission, but the logical follow-on is a mission to investigate the closest stars to learn whether habitable planets are to be found within 100 light years or less, and ultimately to gather spectroscopic data about their atmospheres. The discovery of an interesting world 4.3 light years away would encourage these plans and, in the long term, energize the idea of interstellar probes. But the near term is now, and we may have results on Centauri A and B in short order. Please join in the effort to fund Debra Fischer and team as the Alpha Centauri hunt continues.

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jkittle April 19, 2012 at 11:08

I suspect there are planets closer in around Centauri A and B but i am concerned that they may be difficult to detect. Almost certainly they are in some sort of resonance with the binary star orbit and thus their signal will be locked in the the massive signal from the binary. Thus we can likely rule in or rule out planets that are NOT in resonance with the pair but it may be hard to detect the more likely case…. I admit I may be wrong about this. i just spend a lot of time designing experiments and learn to look for problems.
What about the investigation of proxima centauri?

henk April 19, 2012 at 11:39

The system is a valuable target whether or not we find a habitable world (or even two) there

alpha centauri can even have 2 planets with life. It is really the best system to find for life.

It is not if we find a habitable world there, it is more like when we are going to find a habitable world there. I think the chance is almost 100% that we are going to find a earth size habitable planet there.

i can not wait when they will find a planet there.
Alpha Centauri is the first solar system that we humans will colonize.

ljk April 19, 2012 at 11:50

Proxima Centauri, the star closest of all to Sol that we know of, does not appear to have any planets even just a few times larger than Earth:

http://www.pagef30.com/2008/07/are-there-planets-around-proxima.html

Of course since red dwarfs constitute the vast majority of stars in the Milky Way galaxy, a mission to PC would certainly be valuable in its own right. And I would be surprised if there were not at least planetoid-type belts of debris around that star.

Rafal April 19, 2012 at 13:30

Forgive me perhaps a naive question, but…
Isn’t Proxima Centauri the closest star? Perhaps for some reasons it is not good candidate for hosting a second Earh, but I find it odd that you haven’t even mention it.
Another question that came to me is: is it possible for a planet to orbit not Centauri Alfa or Beta but both of them together (or their centre of mass) at some larger distance. Then we wouldn’t have the issue of unstable orbits.

Mike April 19, 2012 at 14:12

Oh well, there goes my beer budget. But I just couldn’t pass this one up either. Anyone know James Cameron’s email address? He made a shit-load of
money off Avatar. You never know. If he knew about the efforts to detect planets at Alpha Centauri maybe he would be willing to pony up a healthy donation toward Dr. Fischer’s 20 day observing run.
By the way Paul, do you know which telescope her team is using for this run?
Is it the same telescope they’ve been using for for the previous 2 years?

Paul Gilster April 19, 2012 at 14:33

Rafal asks:

Isn’t Proxima Centauri the closest star? Perhaps for some reasons it is not good candidate for hosting a second Earh, but I find it odd that you haven’t even mention it.
Another question that came to me is: is it possible for a planet to orbit not Centauri Alfa or Beta but both of them together (or their centre of mass) at some larger distance. Then we wouldn’t have the issue of unstable orbits.

Rafal, Proxima Centauri is indeed the closest known star at 4.2 light years, and it is evidently loosely bound to Centauri A and B. Thus far planet searches there haven’t turned up any, though we’re getting to the point where we can exclude larger planets. More on this in an upcoming post. Most of the attention goes to Centauri A and B just because both are similar to the Sun and we might actually have two planetary systems to work with. But of course if an interesting world turns up around Proxima, it would be a future target for observation and, one day, perhaps a probe.

Incidentally, Beta Centauri is not Centauri B. The Alpha Centauri system consists of Centauri A, Centauri B and Proxima Centauri. Beta Centauri is an entirely different star, the second brightest as seen from Earth in the constellation Centaurus. Anyway, the idea of a planet orbiting both the A and B stars is something we’ve seen in a few other systems, but I haven’t seen any speculation on this in the literature. Maybe some of the readers have.

Paul Gilster April 19, 2012 at 14:35

Mike asks:

By the way Paul, do you know which telescope her team is using for this run?
Is it the same telescope they’ve been using for for the previous 2 years?

I’d be willing to bet the beer money that it is the same, but I don’t have confirmation — will look into this.

FrankH April 19, 2012 at 16:02

The orbits of Alpha Centauri AB around the system barycenter is pretty elliptical. A planet in a STABLE orbit around the pair would have to be far beyond the semi-major axis; we’re taking 45- 50AU or more. It’ll be a cold, cold place.

Proxima Centauri is not only very faint (intrinsically) but also a flare star. If it has any planets in its habitable zone, they’re definitely not going to be habitable during a flare.

The main reason Alpha Centauri B is considered a better candidate is because its spectra is quieter than that of Alpha Centauri A. The very small changes in radial velocity expected from an Earth-sized planet would be easier to detect. It’s also less massive than A, so an Earth-sized planet would produce a slightly greater signal.

Kenneth Harmon April 19, 2012 at 18:10

So if Henk is wrong and we find nothing of interest over the next 10 years around either Alpha Centauri A or B (or Proxima Centauri) then what? How about Epsilon Indi, Tau Ceti, or a couple of other candidates in the 10 Ly-12 Ly “plan B” area? If nothing there then we may have to go all the way out to around 19-21 Ly’s where there seems to be a whole slow of interesting candidates. Unfortunately, once we are beyond about 12 Ly’s from Sol/Terra the prospects for a manned or unmanned interstellar probe within the next ~200 years (and sadly perhaps even ~500 years) seems to decrease dramatically unless of course there is some sort of major breakthrough in the area of “new physics” which completely changes the game. If we find something of interest (i.e habitable planet) within ~12 Ly’s of Sol/Terra then we may have a chance to go relatively early since there will be “demand pull” ($1-10 Trillion for a new World assuming it is not already occupied), which could be compelling.

The farther in distance the “demand pull” of a habitable World the stronger will be the argument that we should stick to Interplanetary Exploration and Exploitation until there is some sort of game-changing physics breakthrough, which could be right around the corner or could take Centuries. Lets hope that Henk is right, because if he is not, and we are stuck with going at .1C average velocity for a whole range of reasons for the foreseeable future then the only Interstellar Traveling we are likely to be doing for many Centuries to come will be through remote observation.

Interstellar Bill April 19, 2012 at 20:17

Alpha Centauri’s prime interest is its magnificent proximity,
with a three-fer bonus as an interesting trinary.
Even without planets it’s bound to have enough asteroids
for vacuum prospectors to make a living.
The only question is who gets there first and how?
Alpha is so close that the early tortoise beats the late hare.

If a 50 year manned voyage begins in 2100
it’s very unlikely that a ten-year voyage will be possible by 2140,
if for no other reason than the ten-year version
probably costs 5-10 times more than a 20-year mission,
which has a similar factor over the 50-year mission,
and this is for all speeds having the same maturity,
but actually the higher speeds will be decades behind the slower.

By ten light years this factor becomes too weak to matter,
so Alpha is special because of its unique tortoise-first status.

As for planets, the system’s low tilt of 9 degrees
makes transits more likely.
Eventually a dedicated hyperscope will find the planets.

A life-bearing world would of course be most exciting of all,
but my odds on that are a thousand to one against, alas,
but high for planets, however few and small.

Gregory Benford April 19, 2012 at 20:20

This is exciting–a clear use for some money to further our agenda.

Dave Moore April 19, 2012 at 22:14

Mike, The team hope to use to Cerro Tololo observatory in Chili. It has a 1.5 meter mirror, which is considerably smaller than the 2.4 meter mirror they use at Lick, their home base, but this is not a problem as Alpha Centauri is so bright that anything larger is superfluous.

Phil April 19, 2012 at 22:42

The HARPS team is looking at Tau ceti. Its a quiet star RV wise, and I think they have already ruled out giant planets, but that doesnt mean there arent earth’s hiding in the sub 1mps noise.

P

A. A. Jackson April 19, 2012 at 23:03

Glad you said ‘interstellar target’.
Because Issac Asimov got quite mad at some school kid who , when he asked the Good Doctor the closest star and got Alpha Centauri…. kid corrected Asimov … with …. the Sun.
(It’s an old trick.)

Istvan April 19, 2012 at 23:59

Very little would catalyze global interest in interstellar projects and space exploration in general more than detection of a world – especially an earthlike world – around a star in the Alpha Centauri system. I’ve thrown what pittance I can this month at that funding page. Any little bit will help the team reach their goal.

Many thanks to Paul Gilster for communicating this opportunity to the Centauri Dreams community. It’s a rare day when we as individuals have a chance to make a small difference in the projects that help humanity progress toward our dreams.

Ronald April 20, 2012 at 5:41

Further to Kenneth Harmon, if we find nothing of interest around either Alpha Centauri A or B: the next more or less solar type stars, Tau Ceti at almost 12 ly, has very low metallicity and hence possibly failed planetary system (lots of dust and rocks), and Epsilon Eridani at over 10 ly, is rather dim for a solar type star, and very young (0.4 – 0.8 gy).

Beyond 16 ly up to 20 ly it gets a bit better with several solar type candidates, which are, however, either rather dim, low-metallicity, variable or moving up in the main-sequence, part of a (close) binary system, or a combination of these.
Around and beyond 20 ly up to about 30 ly it gets really interesting with a few of my favorite candidates: 82 Eridani, 61 Virginis, Beta Canum Venaticorum, Alpha Mensae.

It is going to be a very exciting 1 or 2 years with regard to planets around Alph Cen A and B. We know that planets in stable orbits can exist around rather close binaries down to about 15 AU, but the closest approach of Alph Cen A and B at 11 AU is just annoying. I mean, computer simulations have shown that stable planetary orbits can indeed exist in their inner systems once formed, but at the same time their initial formation is at question. I suspect that A, being of large mass and high metallicity, if it it has any planets, it may have a compact inner system of super-earths and Neptunes, and well inside the inner edge of its HZ at about 1.2 AU.
My highest hopes are on B, since various computer simulations have show that a terrestrial planet may have formed in a stable orbit inside its HZ from 0.5 – 0.9 AU (Xie et al. 2010).

Ronald April 20, 2012 at 5:57

Further to my previous comment, on Alph Cen A: besides, because of its large mass (about 1.1 * solar) and estimated higher age (at least 4.5, but probably 5.5 – 6.0 gy), though still being on the main sequence for a while, it has been getting increasingly brighter, presently 1.56 * solar luminosity. This means that any planets that originally formed and developed inside the HZ must now be far too hot.
B, a K0/1 star of about 0.9 * solar mass and only about 0.45 times solar luminosity, does not have that problem yet and not for a long time to come, i.e. its continous HZ is long-term very stable.

jkittle April 20, 2012 at 9:09

interstellar bill
nice comment
by there way.. on the topic of new physics
There will be new physics because we have reached the edges of the current paradigm. It is simply not possible to account for the signature of Dark energy or Dark matter without some “new Physics” the question is: will it just be a simple extension of what we currently theorize or will it really cause us to rethink the basics just as relativity and quantum mechanics did not invalidate earlier work but caused a total reinterpretation. The next questions are -when will this new physics emerge and will it improve our ability to build a star ship or make it clear the goal will require long voyages in ships using lots of resources to build and launch

ljk April 20, 2012 at 10:18

A. A. Jackson said on April 19, 2012 at 23:03:

“Glad you said ‘interstellar target’.
Because Issac Asimov got quite mad at some school kid who , when he asked the Good Doctor the closest star and got Alpha Centauri…. kid corrected Asimov … with …. the Sun. (It’s an old trick.)”

Ray Bradbury had a supposedly similar incident with a kid who corrected him on which direction the two moons of Mars rise over that planet in his famous Martian Chronicles. His response would be considered not very PC today.

Interstellar Bill April 20, 2012 at 10:31

jkittle, thanks for the appreciation but I disagree with you on new physics.

Dark matter is entirely hypothetical, posited merely to save our concept of gravity from 1) non-Keplerian stellar orbits in galaxies; 2) non-virial motions of members of clusters of galaxies. See Halton Arp for a full critique. Dark matter is for lazy people who can’t figure out gravity. Forget new physics and just get rid of the old, wrong physics of gravity-only between stars, and look for the action of electric fields as well.

Dark energy is entirely hypothetical as well, positied to save the concept of the expanding universe with everything created at the same time. Dark energy is invoked to explain away the galaxies that are older than ours and thus have a smaller red shift than predicted from their distance. What is an actual disproof of the expanding universe is turned into an ‘acceleration’ – Presto! The spirit of epicycles lives on.

The only place to look for new physics is in actual experiments with accelerators and cosmic rays. So far, no extra dimensions or FTL, and absolutely no experimental indication they will ever be found. If there was any ‘new physics’ it would have shown up somewhere by now.

Besides if FTL was possible the entire Universe would have been overrun billions of years ago, making the Fermi Paradox a trillion times more paradoxical.

Mike Lockmoore April 20, 2012 at 13:35

I’m currious how big of a primary mirror or interferrometer spacing we would need to resolve a planet at Alpha Centauri B’s habibility zone range. I’m not talking clearly resolving a planetary disk, just enough to distinguish an orbiting companion at 0.4 AU or more, assuming good enough ocultation of A. Centauri B itself, of course. Maybe a scaled-down Terrestial Planet Finder, since that was not funded. At least enough resolution to determine orbital period, maybe enough to do some spectroscopy? Of course, if it is to work for other stars out to 20 LY, it would need to be about four times bigger.

OK, I tried to estimate the angular separation… if I did it right, a separation of 0.4 AU at A Centauri’s distance is as much as 0.3 arcsec. Is this right? We’d only need an aperature of a meter or so!?! Well, pragmatically, we’d probably need a lot more than this to effectively block the primary and be able to see a small, faint planet. Has Hubble tried?

Ron S April 20, 2012 at 15:24

Bill: “Dark matter is for lazy people who can’t figure out gravity.”

Apart from a discredited theory by Arp can you back up this broadside insult against the peer-reviewed, **evidence-based** work of thousands of “lazy” scientists?

FrankH April 20, 2012 at 16:59

Ron,
Halton Arp is about as crank-y as a “real” astronomer gets. However, even a broken clock is right twice a day and it ‘s becoming clear that the huge amounts of dark matter postulated by the leading theories just can’t be right.

Here are a couple of good articles:
The dark matter crisis: falsification of the current standard model of cosmology
Pavel Kroupa
http://arxiv.org/abs/1204.2546

And one based on real data:
Missing Dark Matter in the Local Universe
I. D. Karachentsev
http://arxiv.org/abs/1204.3377

MOND (or some variant, like TeVeS) and some dark matter in the form of neutrinos can pretty much match observations from galaxies through clusters.
This page has real data and comparisons with CDM:
http://www.astro.umd.edu/~ssm/mond/index.html

Rob Henry April 20, 2012 at 19:59

FrankH, I think you are spot on. I think that we have yet another case here of science being distorted by its paradigm approach. We have anomalies that can be fixed by postulating the existence of dark matter and dark energy. In each case these dark entities might be the simplest and most elegant solutions to these problems, and the ones that least compromise our current models. Hell, I could even stretch my mind to believe that these were the most likely solution to the dilemma given current data. Where we have failed is in our (implicit?) belief that these solutions to the anomalies are more likely than every alternative theory combined. I don’t see how in any period to date, the data on any supposed direct evidence for their existence could have ever been seen to be sufficient to support that view.

Joy April 20, 2012 at 22:11

@Mike Lockmoore – Hubble has an angular resolution of 0.05 arcsec for two objects of equal luminosity. The problem with picking a planet out from a star is the huge difference in luminosity, and the Airy disk (or Airy pattern) made by the star. Even the outer dim rings of the Airy pattern from a star would wash out the light of a planet. Wikipedia has a decent article on the (cancelled) Terrestrial Planet Finder program, which describes the types of space telescopes required for actual direct planet imaging. The Visible Light Coronagraph (TPF-C) probably could be built, for a cost significantly higher that the troubled $8. 7 billion Webb IR telescope, which might launch in 2018. I doubt anything larger than the Webb will fly before 2030 in any case. In theory, the Webb could still be paired with a separate $3 billion starshade satellite (New Worlds Mission) to give it some planet detection capability, but there is no plan to actually do this, just vague hopes some gigabucks donor will appear. I understand Warren Buffet (Forbes net worth $44 billion) has prostate cancer, but AFAIK he has no interest in astronomy. Makes you appreciate the good value of ground based radial velocity planet searches.

Tarmen April 21, 2012 at 13:03

We can’t be sure there are no brown dwarf systems closer than Alpha Centauri system. Such a place might provide useful planets and moons. I’ll assume that our future bio-habitat engineers can make use of many non-earth-like worlds.

Unfriendly worlds that are within reach are still going to be useful for some post-human economies based on ideas beyond the pale of Earth-based humans today. I mean genetically altered people and animals and cyberneticly enhanced people and animals. And autonomous robot beings created by such. All material and energy can be useful in some way by some kinds of our descendants. Some of our descendants’ methods would be considered evil by today’s judgment. But who will argue with success? Sci-fi suggests that someone may. And war will come with us into the stars.

Wojciech April 21, 2012 at 13:19

Contacting people like Cameron, while it may seem fantasy, could be possible, especially if it is done through Planetary Society which has some more media connected members. People like Cameron, while elusive, do make some public appearances from time to time, and such a question could be issued to them. I do believe however that for scientists from PS other channels are available, which would be more effective.

Another question:would it be possible to launch a single-purpose satellite whose aim would be to observe just one star for planets? Would it be cheap or would it have to cost hundreds of millions? I am thinking about low-cost mission just for the goal of discovering planets around one star like Alpha Centauri B.

andy April 21, 2012 at 15:19

Tau Ceti at almost 12 ly, has very low metallicity and hence possibly failed planetary system (lots of dust and rocks)

The metallicity of Tau Ceti should not be a particular problem: we already know of several exoplanetary systems at comparable metallicities, including Gliese 667 C and its potentially-habitable super-Earth.

Dust discs should be good indicators of a star that hosts terrestrial planets: the types of dynamical instabilities among giant planets that would be harmful to planet formation in the inner part of the solar system are also really efficient at clearing out the debris that produces the dust disc. See this paper and the follow-up. There’s no real reason to think that Tau Ceti didn’t form planets, however it is probably not a good place to go hunting for gas giants.

The real problem with Tau Ceti is the age of the system, it has been around long enough that the habitability of Earth-like planets there may have collapsed due to the geological activity of the planet dying down.

Ron S April 21, 2012 at 15:55

FrankH, Rob,

You are free to believe as you will notwithstanding selective quotes from the literature and elsewhere. However if you are ready to dig into those alternatives and “paradigms” you will find that they are largely discounted based on the evidence. That some cling to falsified or unlikely scenarios is not persuasive.

While this point is not directed at you, I am frequently amused that many of those who desperately wish for new physics also have an aversion to phenomena such as dark matter which may promise just that. What it often comes down to is that some people are just inveterate contrarians.

jkittle April 21, 2012 at 23:07

iBill
It turns out that your dismissal of big bang or dark energy and dark matter is also a possible direction of “new physics” I define new physics as that which is clearly more than an extension of the existing Canon. You dislike the expanding universe model- fair enough. It is core to existing cosmology so you are proposing new physics. Recall I did not assume that ” new physics” will provide a means to improve interstellar travel- just that new physics is coming and i hold out the possibility.
Finally it is all about the math- if you can create the math to account for observed phenomena then you are the man ( or woman) in charge! all else is just speculation.

Joy April 22, 2012 at 0:36

Wojciech – “question: would it be possible to launch a single-purpose satellite whose aim would be to observe just one star for planets? Would it be cheap or would it have to cost hundreds of millions?”

Easy answers –
1) NO, there is no cost savings at all in only observing one star. You still have to build a fully capable telescope.
2) Hundreds of millions is very cheap for a space telescope! Something capable for finding terrestrial planets will not be cheap. It has been estimated that a 2 meter (Hubble/spy satellite class) space telescope equipped with a vortex coronagraph could detect an Earth twin (1 AU from a sun-like star) out to 30 light years. The last space telescope of that size class (with a bigger than Hubble 3 meter mirror), USA-224, was launched on 20 January 2011 by a Delta IV Heavy rocket and cost about $4.5 billion. There would be some additional cost to adapting a spy satellite design for astronomy – as was done with Hubble. The instruments would all have to be purpose built. It would probably cost around $1o billion in the end, money that NASA doesn’t have, and is unlikely to have in the foreseeable future.

Fortunately, there is still potential for upside surprises in ground based astronomy. Truly vast mirrors plus advances in adaptive optics and the vortex coronagraph might eventually allow ground based detection of terrestrial planets.

Ronald April 23, 2012 at 6:13

@andy: you are right about low metallicity and the possible presence of terrestrial planets (I actually had both publications already), at least beyond a (apparently very low) threshold metallicity.

I find particularly the following sentence very relevantt: “We predict an anti-correlation between debris disks and eccentric giant planets, and a positive correlation between debris disks and terrestrial planets.”

Furthermore they say that a system with very (a) large giant planet(s) and/or more than one equal mass giant planets has a great chance of not having terrestrial planets because of this resulting eccentricity.

It is also known from HARPS research that there is a very significant positive correllation between metallicity and giant planet abundance. This could mean that beyond a certain metallicity giant planets become too large and/or abundant for terrestrial planets to survive. In that case very high metallicity would actually be a bad thing for terrestrial planets!

Ronald April 23, 2012 at 6:15

@Joy: “Fortunately, there is still potential for upside surprises in ground based astronomy. Truly vast mirrors plus advances in adaptive optics and the vortex coronagraph might eventually allow ground based detection of terrestrial planets.”
This would be great, however: wouldn’t the earth atmosphere always pose a problem with regard to infrared absorption?

Holger April 23, 2012 at 13:11

@Tarmen: “We can’t be sure there are no brown dwarf systems closer than Alpha Centauri system.”
Unfortunately we can be quite sure: As Paul Gilster posted here 1-2 weeks ago (“Re: WISE”), no new brown dwarfs closer than 10 ly have been found by WISE. And WISE was supposed to find all brown dwarfs with T>150 K within 10 ly (which is already much too cold for liquid water to exist on the star’s surface(!), let alone on its planets). Even if some of the found dwarfs should turn out to be somewhat less than 10 ly away with additional data, it seems extremely dubious for one to actually be closer than 4.3 ly.

Paul Gilster April 23, 2012 at 14:15

Holger, sorry if I wasn’t clear. My source told me there have been no new brown dwarf detections within 10 light years yet — the search continues, but nothing has turned up to this date. There are still a lot of data to comb through.

Holger April 24, 2012 at 7:50

Paul, thanks for clarifying. I was under the impression that they had already combed through all the data, at least “preliminarily”, since they have already released the whole data to the scientific community.
So do you know when we can expect the WISE team to give a definite answer to the question of whether they found nearby brown dwarfs? It’s already 18 months sice the primary mission ended…

ljk April 24, 2012 at 9:32

And let us not forget that there may be plenty of rogue planets out there sailing quite close to the Sol system, closer than any known star of any type.

Of course detecting them is going to be quite a trick, as I can imagine their infrared signatures are pretty minute. Any chance of watching for dark objects moving in front of stars in a path? I bet a Worldship would radiate more heat than a rogue world.

Paul Gilster April 24, 2012 at 9:37

Holger writes:

So do you know when we can expect the WISE team to give a definite answer to the question of whether they found nearby brown dwarfs? It’s already 18 months sice the primary mission ended…

It’s hard to say, Holger, but I’ll check back with my WISE source soon to see if anything else has turned up. I’ll post anything I can find out.

Ron S April 24, 2012 at 10:44

ljk: “Any chance of watching for dark objects moving in front of stars in a path?”

There have been searches for dark objects that gravitationally enhance stars in globular clusters and the Magellanic clouds. Those are good test backgrounds because of the arc density of stars that can be observed. I believe that this was only intended to test for MACHOs as dark matter candidates. Eclipses of rogue planets, and even brown dwarfs, may be within the noise or error bars, or at least I am unaware of such searches.

Carlos A. Santos September 3, 2012 at 14:38

Great to hear on the effort in trying to find planets around Alapha Centauri, but the absence of gas giants makes it perhaps hard to find rocky worlds, gas giants tend to shield rocky worlds from astoired impacts. But it is very intriging though, Alpha Centauri is a sun like star, as well as Centauri B, albeit smaller than the Sun; as SETI ever tried to listen to radio signals from the Alpha Centauri? if Kepler tried turned its eye on the Centauri system, would detection of planets be quicker?

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