We’ve often speculated here about how many stars exist in the Milky Way. Earlier estimates have ranged from one hundred billion up to four hundred billion, with a few wildcard guesses in the range of one trillion. The number is still, of course, inexact, but recent work has led to a serious misunderstanding of the subject. As reported in this earlier post, Harvard’s Mark Reid and colleagues have discovered that the Milky Way is likely to be as massive as the Andromeda galaxy, which means that it could have the mass of three trillion stars like our own Sun.
Does that mean that the Milky Way contains three trillion stars? Absolutely not. I’m seeing the three trillion star number popping up all over the Internet, and almost reported it that way here when I first encountered the work. The misunderstanding comes from making mistaken assumptions about galactic mass. Reid used the Very Long Baseline Array to examine regions of intense star formation across the galaxy, a study the scientist reported at the American Astronomical Society’s winter meeting this past January. The Milky Way does indeed turn out to have much more mass than earlier studies had indicated.
Mass on the Galactic Scale
Science News ran a story on Reid’s work with this headline: “This Just In: Milky Way as Massive as Three Trillion Suns.” The headline is catchy, but read it carefully. It does not say the Milky Way contains three trillion stars. What it does say is that the galaxy has been found to be as massive as three trillion suns. In other words, it has the mass, at the upper limit, of three trillion G-class stars like the Sun.
Now factor in our understanding of galactic mass. Current thinking says that dark matter accounts for nine-tenths of a galaxy’s mass, and perhaps more. What Reid’s work shows us is that the galaxy is massive indeed, about fifty percent heavier than previously thought. But bumping up the mass estimate also bumps up the estimate of dark matter, and does not imply that the galaxy contains three trillion stars, or anywhere near it. Again, most of the mass in a galaxy is in the form of dark matter, which is what most of that additional mass would be made of.
The Dominance of Dark Matter
Because I wanted to be sure I had this right, I wrote to Dr. Reid asking whether the idea that the galaxy contained three trillion stars wasn’t a serious error, one that did not follow from his work. He agreed:
What we’ve found is that the Milky Way is rotating about 15% faster (254 km/s) than previously assumed (220 km/s). The faster rotation speed matches that of the nearby Andromeda galaxy (M31). The simplest interpretation is then that the Milky Way and Andromeda have a similar overall mass and size (that is dominated by dark matter halos). Estimates of the total mass of Andromeda are typically between 1 and 3 trillion solar masses. However, only a small fraction of that is in normal matter (eg stars), probably only about 0.1 trillion [italics mine].
A tenth of a trillion — a hundred billion — solar masses is what we have to work with in terms of normal matter. That figure would then need to be adjusted to reflect the relative abundance of stellar types, from the tiniest brown dwarf to the largest blue giant, to get a star count. There is still room for a range of estimates, but we’ve learned that while the Milky Way is crammed with stars, it’s a long way from having three trillion of them.
George Dvorsky takes on the ‘rare earth’ hypothesis in his Sentient Developments blog, calling it a ‘delusion’ and noting all the reasons why life in the galaxy is unlikely to be unusual. The post reminds me why the book that spawned all this was so significant. Rare Earth: Why Complex Life is Uncommon in the Universe (Copernicus, 2000) is Peter Ward and Donald Brownlee’s take on our place in the cosmos, concluding that complex life is rare because an incredibly fortuitous chain of circumstances must arise for it to occur. Indeed, the authors argue that large parts of our galaxy are composed of what they call ‘dead zones.’
The argument is complex and looks at factors ranging from a planet’s place in the galactic habitable zone (itself a controversial subject), its orbit around its own star, its size, its satellites, its magnetosphere, its plate tectonics, and more. I’m surprised to realize, looking through our archives here, that I haven’t managed to do a complete review of Rare Earth in the past five years (although we’ve certainly kicked its ideas around during that time). I’ll remedy that soon, because I think it’s a hugely significant work. Agree with it or not, the authors have laid out a strong case that has inspired spirited rebuttal, and the study of astrobiology has benefited from the insights they’ve brought.
Back to Dvorsky, who notes Charles Lineweaver’s work suggesting that planets began forming in our galaxy long before our own Sun ever ignited, and goes on to look at progress in exoplanetary science, which is uncovering so many new worlds. The point George is making is that there have been vast amounts of time for life to arise, and a huge range of galaxies within which it could evolve. He’s absolutely right, but I’ll part with him on the use of Gliese 581 c to make the case on life’s odds:
…shockingly, the first Earthlike planet was discovered in 2007 orbiting the red star Gliese 581. It’s only 20 light-years away, 1.5 times the diameter of Earth, is suspected to have water and an atmosphere, and its temperature fluctuates between 0 and 40 degrees Celsius.
If we are one in a billion, then, and considering that there are only 0.004 stars per cubic light-year, what are the odds that another Earthlike planet is a mere 20 light-years away?
But the odds are still where they were, Gliese 581 c now being generally considered too close to its star to be habitable. The next planet out, Gl 581 d, is possibly within the extreme outer edge of the habitable zone, and perhaps a better bet for life than what now seems to be a hellishly hot Gl 581 c, but in any case neither of these massive ‘super-Earths’ are truly Earth-like, and the question of whether they support life or not awaits further data. Recent work by Brian Jackson, Richard Greenberg and Rory Barnes suggests tidal heating for Gl 581 c that may be three times greater than what we see on Io, not a promising sign.
We still, in other words, don’t have a good read on how close the nearest habitable planet is, or how rare our Earth may be. Could Ward and Brownlee still be right? Absolutely. At this point we lack the data to know.
Alan Boss has much to say about all this, and George goes on to quote him to good effect. Author of the recent The Crowded Universe: The Search for Living Planets, Boss (Carnegie Institution) sees Earths as ‘incredibly common,’ going on to make this astounding statement: “…every solar-type star probably has a few Earth-like planets, or something very close to it.” Take that, rare Earthers!
I just finished The Crowded Universe the other day and will have more to say about it soon — we looked at its author’s ideas on habitable planets in a previous post. I tend to go along with Boss, as least in so far that I suspect life is quite common elsewhere in the universe, and probably complex life at that. But the question of sentient, technology-building life is quite another matter. It wouldn’t surprise me at all to find many living worlds in our galaxy, but of necessity we await the results of our explorations, one of which was so recently launched. One day Kepler’s successors will doubtless flag the spectroscopic signature of a living world, but barring a SETI breakthrough, the question will remain. Are there other civilizations? Do they too dream of traveling to the stars?
We now have on the order of 335 confirmed exoplanets, with an ongoing race between the CoRoT and Kepler teams to find the first Earth analog in the habitable zone around another star. CoRoT’s shorter observation cycles make finding a terrestrial world around a G-class star problematic — the orbit would necessarily be on the order of a year, and the transit would then have to be confirmed with additional transits and whatever radial velocity observations could be mustered. But CoRoT just might find an Earth-class planet in the habitable zone of a K-class star, so we shouldn’t assume Kepler is necessarily going to win the ‘habitable Earth’ race.
I mentioned a few days back that the Planetary Society has unveiled its new Catalog of Exoplanets, a fine resource with the basics on detection methods and a glossary that complements a catalog filled with helpful orbital animations. If you want to get a quick read on a given exoplanet, take a look here. Some of these planets have gone beyond the stage of being purely numerical designations to emerge as places in their own right, with our deepening knowledge making them come alive. How long will it be before we start naming these worlds?
Consider HD 189733 b, some 62 light years from us, a hot Jupiter orbiting at a distance of some 5 million kilometers from its star. Because we found this one (four years ago) by radial velocity methods and then were able to pull off a transit detection, HD 189733b has become one of the best understood exoplanets we’ve studied. Who would have thought, back in 1995 when Michel Mayor and Didier Queloz identified 51 Pegasi b, that within fifteen years we would be creating an absorption spectrum of an exoplanetary atmosphere, revealing gases like water vapor, methane and carbon dioxide, mixing with potassium, sodium and ammonia?
The Catalog of Exoplanets has a section that runs through the most notable finds thus far, including the 55 Cancri system, where five planets, all of them gas giants, orbit the star with the most exoplanet detections. Here we’re in interesting territory in terms of the habitable zone, because 55 Cancri f is a Saturn-sized planet near the zone where water could exist on a planetary surface, leading to inevitable speculation about rocky moons and their potential for life. And then there’s Gliese 581 c, well known because of its nearness to the habitable zone and the media attention that followed its discovery, along with icy ‘super-Earth’ Gl 581 d.
Image: The system around 55 Cancri in comparison to our own. Credit: NASA/JPL-Caltech.
What I’m thinking is that a browse through the Catalog of Exoplanets now is going to be memorable when you consider what will be in the catalog within a decade or so. Sort of like jumping from site to site back in the early days of the Web, when the explosive growth of commerce and search engines and personal pages had yet to occur. How fascinating and yet sparse the Web was then, and how much richer it has become. Will the Catalog of Exoplanets a decade hence list hundreds of terrestrial planets, many in the habitable zone of their stars? The odds seem to favor it, though our planet hunter spacecraft will have the next word.
And a final thought: Given the budgetary problems that have beset the Space Interferometry Mission (now known as SIM Lite, a change of name that speaks volumes) and the mutating Terrestrial Planet Finder concepts, Kepler has emerged as the NASA planet hunter with the most to prove. If it turns out that we find few terrestrial worlds where we think there should be many, the impetus for more advanced missions to characterize planetary atmospheres, for example, is bound to be weakened no matter when the economy begins to recover. Let’s hope Kepler and CoRoT both yield a continuing harvest of small new worlds.
Take a look at NGC 4565, a spiral galaxy seen edge-on. Spiral galaxies viewed at this angle often show dark dust lanes, the result of dust from dying stars mixing with interstellar gas. We’ve discussed the problem of interstellar dust in terms of objects moving at relativistic speeds between stars, but recent quasar studies are showing us that entire galaxies may expel dust to distances of several hundred thousand light years. In terms of the NGC 4565 image, that would be ten times farther than the visible edge of the galaxy.
Image: Spiral galaxies seen edge-on often show dark lanes of interstellar dust blocking light from the galaxy’s stars, as in this image of the galaxy NGC 4565. The dust is formed in the outer regions of dying stars, and it drifts off to mix with interstellar gas. Credit: Sloan Digital Sky Survey (SDSS-II).
The astronomers who did this work talk of intergalactic space being filled with a haze of fine dust particles, a haze that can be examined by analyzing light from distant quasars as it moves near foreground galaxies on its way to Earth. The colors of 100,000 quasars were studied via the Sloan Digital Sky Survey, their light examined in relation to some 20 million galaxies. The reddening of quasars from intergalactic dust draws on the fact that dust grains block blue light more effectively than red, an effect commonly seen in sunsets on Earth.
So is the reddening of quasars caused by intergalactic dust a showstopper for dark energy, that still mysterious effect seemingly responsible for the acceleration of the universe’s expansion? Evidently not — the effects are simply too subtle. Says Ryan Scranton (UC-Davis): “Our results imply that most distant supernovae are seen through a bit of haze, which may affect estimates of their distances.” No showstopper here. Scranton simply describes the dust as ‘a bit of a nuisance,’ one that has to be factored into future high precision measurements as we try to understand how dark energy is changing the cosmos.
Much depends on the result. From the paper (internal references omitted for brevity):
Light rays from distant sources carry unique information about the matter and gravitational potential along the line-of-sight. A well-known example is the signature of intervening gas clouds imprinted into spectra of background sources via absorption lines. Mass concentrations located along the path of photons can also induce gravitational lensing effects. Background sources can be magnified… and galaxy shapes can be distorted as measured through galaxy-galaxy lensing… and cosmic shear. Measuring these effects has become a powerful tool for probing the mass distribution in the Universe.
The paper is Ménard et al., “Measuring the galaxy-mass and galaxy-dust correlations through magnification and reddening,” submitted to Monthly Notices of the Royal Astronomical Society and available online.
A nice, tidy liftoff for Kepler, and like all night launches, well worth watching. The mission is generating a satisfying amount of attention in the press and a slew of news releases, from one of which which I’ll quote Geoff Marcy:
“In part, learning about other Earths — the frequency of them, the environment on them, the stability of the environment on other Earths, their habitability over the eons — is going to teach us about our own Earth, how fragile and special it might be. We learn a little bit about home, ironically, by studying the stars.”
And of course it’s hard to argue with that, although the focus for most of us will only be tangentially here and most emphatically there — just how many terrestrial worlds are out there, and how likely are the chances for their being in the habitable zone? Marcy gets preferential treatment here simply because, along with Paul Butler and a team of exoplanet hunters spread out over the globe, he has been involved in almost half of our exoplanet detections.
As we sit back to monitor Kepler’s progress, enjoy a bit of weekend reading with the latest Carnival of Space, offered through the good services of Emily Lakdawalla at the Planetary Society Blog. Be aware of the Society’s new Catalog of Exoplanets, designed for anyone with an interest in these matters from the rankest amateur to student and professional, and note the helpful animations, where you can see planets in orbit around their stars. It’s a user-friendly site that should do much to keep the planet hunt accessible to the public.
More reading for the weekend might include Charlie Stross’ 21st Century FAQ, wherein the futurist condenses the next ninety years into a few paragraphs guaranteed to cause controversy. As in this statement about space colonization:
Forget it.
Assuming we avoid a systemic collapse, there’ll probably be a moon base, by and by. Whether it’s American, Chinese, Indian, or Indonesian is anybody’s guess, and probably doesn’t matter as far as the 99.999% of the human species who will never get off the planet are concerned. There’ll probably be a Mars expedition too. But barring fundamental biomedical breakthroughs, or physics/engineering breakthroughs that play hell with the laws of physics as currently understood, canned monkeys aren’t going to Jupiter any time soon, never mind colonizing the universe. (See also Saturn’s Children for a somewhat snarky look at this.)
I see that Brian Wang has taken on Stross on his NextBigFuture site, Brian being a proponent of Orion technologies that scale up to massive spacecraft that could theoretically open up the outer Solar System to human exploration. Re Orion, John Hunt coincidentally passed along this video, the first of six available on YouTube on the subject, drawn from a BBC show called To Mars by A-Bomb. The first segment is below:
But I think, Orion or not, that Charlie should be taken seriously. He’s talking about the 21st Century, the start of which should remind us that while we’ve been able to do some remarkable robotic missions to places like Saturn, we’re a long way from expanding a manned human presence to another world, even the Moon. If you confine your time frame to this century alone, his position isn’t extreme.
Not that I necessarily agree. The further out we look, the murkier the shape of things, and I’m a long way from being convinced that breakthroughs in artificial intelligence and nanotechnology will not rule out launching some seriously interesting missions (unmanned) to destinations now thought unreachable, such as the outer Kuiper Belt or the Oort Cloud, within the next hundred years. In any case, I have little confidence in technological predictions that attempt to get too specific.
Here’s where I am on this: We should be working within a long-term horizon, mounting an effort to explore space with the understanding that our work will be passed along to future generations. I am relatively sure that we will, by whatever technology, eventually get humans to the stars, but I would be astonished if it happens in this century. That shouldn’t slow down the necessary work of exploring propulsion alternatives and analyzing planetary systems, because what we are after is to understand the universe better, and bit by bit to explore it. Whether achieving an interstellar capability is a matter of centuries or millennia, what counts is that we do what we can to contribute to that goal now.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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