How Many Earths?

At face value, a team of astronomers led by Andrew Howard and Geoffrey Marcy (University of California, Berkeley) has made an audacious claim. Based on observations of 166 nearby stars like the Sun in mass and brightness, the observers calculate that Earth-mass planets should be orbiting roughly a fourth of such stars. But they haven't found a single one of these putative Earths — it's an entirely statistical argument.

Here's the logic behind their conclusions, which appear in tomorrow's issue of Science. These observers have been toiling at Keck Observatory for five years to identify low-mass planets around 230 solar-type stars within about 80 light-years of us. They've been using a tried-and-true technique: noting wobbles in the stars' back-and-forth motion, using sensitive radial-velocity measurements, that betray the gravitational tug of a world in orbit around it. The bigger the wobble, the more massive the planet; the faster the wobble, the shorter the planet's orbital period.

How many Earths?
The sizes of planets around 166 nearby, Sun-like stars show a clear trend: small planets outnumber larger ones. Astronomers extrapolated from these data to estimate that nearly one in four such stars host close-in, Earth-size planets.
NASA / JPL / UC Berkeley
The team's final tally shows 33 planets around 22 of the candidate G- and K-type stars, and about half of these orbit in less than 50 days (putting them within about 25 million miles of their host stars). Odds are good that all the really massive planets, Jupiters on up, have been identified, because they create the most obvious wobbles in their stars. Yet only a few of these were detected.

A higher percentage of the finds fall into the categories of "Neptunes" and "super-Earths", those at least 3 to 30 Earths in mass. But their weaker gravity translates to smaller stellar wobbles, and the team assumes that more than half of the planets in this mass range await discovery.

Anything smaller still, close to Earth in mass, would have escaped detection — the state of Doppler spectroscopy just isn't good enough to spot the anemic wobbles they induce in their stars' motion — and, in fact, the survey didn't turn up a single Earth-class planet. But the trend seems clear: for every "Jupiter" there are many more "Neptunes" and "super-Earths" and, by statistical extrapolation, even more "Earths." In fact, since the survey was limited to close-in (50-day) orbits, the authors argue that even more low-mass planets could be crowding around all those alien Suns if the search zone were extended out to 1 astronomical unit (Earth's orbital distance from the Sun).

You can read more about the search strategy and the team's conclusions in the press releases here and here.

Interestingly, past simulations of planet formation suggest just the opposite, predicting a deficit of planets having 5 to 30 Earth masses and orbital periods of less than 50 days.

So who's right? I'm no statistician, so don't ask me! A better bet would be to wait for the full results from NASA's Kepler mission, now in the midst of staring down 156,000 stars for evidence of planets that transit their stars. Kepler is able to detect Earth-size worlds, and Howard's team estimates that a couple hundred of them (up to twice Earth's diameter) should eventually turn up in the spacecraft's observations of stars like the Sun.

8 thoughts on “How Many Earths?

  1. Dieter Kreuer

    I would be very sceptic about statistics based on extrapolation alone. Just recently, you had an article on new findings regarding planetary formation ( where it was claimed that the terrestrial planets could only have formed because a cosmic dance of Jupiter and Saturn cut short the supply of material for the inner planets during formation. So far, the planetary systems found have the giant planets much closer in, which could mean that their supply had not been cut off (or they drifted in closer than Jupiter, but this would also have destroyed the chance for small planets to form). Of course, the tidal drag measurements that revealed most planets to date are highly biased towards giant planets orbiting close in, as these cause the most visible effects in a limited observation time. But Kepler will also be biased to inner planets, as transits are more likely for close orbits than for wide ones (just compare the frequency of Mercury transit to those of Venus transits). So, whoever is citing statistics on Earth-like planets, is just educatedly guessing. The fact is, we just don’t know, and we should be honest about this.

  2. Tjapko Smits

    The fact with NASA is that they need money for their projects. Billions of dollars are involved and they win the sympaty of the average reader by posting such information that is technically difficult and not based on something that we can eiter see or understand.I am not against this kind of techniques because after all the work they do is admirable.

    Lately everybody seems interested in exoplanets, habitable zones and life beyond our solar system in general.

    The start off point is to find earth sized planets.

    The extrapolation made in the article is based on studies of 166 less than 80 ly sun like stars that might have earth-sized planets.

    Is this assumption based on a extrapolation of only a 166 sun like starts in our own (200 billion stars) milky-way sufficient to conclude we have earth-sized planets like grains of sand?

    The weak anthropic principle states that in a universe that is large or infinite in space and/or time, the conditions necessary for the development of earth-sized planets that can sustain intelligent life will be met only in certain regions that are limited in space and time.
    The intelligent beings in these regions should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that are necessary for their existence or the presence of earth-sized planets.

    What you think?

  3. Dieter Kreuer

    It’s not so much the local sample of observed stars that worries me. Our region in space is not much different from what we observe in most of the rest of this galaxy or others. It’s more the technique of observation which is systematically biased by yet unsurmountable technical limitations. The bias is towards large planets orbiting their parent star rather closely. For Kepler, it’s planets of any size, still orbiting their star closely. This will certainly affect the resulting statistics, so any extrapolation stands on very weak ground. Assume that you were to detect the number of pedestrians in a distant village at night solely from events when they cross the beams of car headlights pointed at you. This happens preferredly at pedestrian crossings. What does this tell you about the actual number of people on the sidewalk? A little, they must be there and cross the street occasionally, but as long as you don’t know how frequently, their total number would be pure guesswork.
    I actually BELIEVE that there are lots of habitable planets out there, there’s just not enough data available to back this, so far. I welcome the research done in this paper, but I doubt the concrete 25% figure, which is based on the assumption of a particular statistical distribution of planet sizes that we just don’t know.

  4. R Holmes

    One problem I have with this article is that the term “Earthlike” is never defined. From context it appears what is meant is that its mass is roughly similar to Earth’s, though it’s not clear how similar it has to be. If this interpretation is correct then I find the term “Earthlike” misleading — Venus for example has an “Earthlike” mass but is far different from Earth in many other ways.

  5. Dieter Kreuer

    I did. “Terrestrial” or “rocky” would be a better term. It’s only about size, and therefore having a solid (or liquid?) surface, so far.

  6. Anthony BarreiroAnthony Barreiro

    The current search for planets around other stars is very exciting, and really in its infancy. One hundred years ago, we didn’t know of the existence of a single galaxy outside the milky way, now we have a whole zoo filled to bursting. Twenty years ago we hadn’t observed a single extrasolar planet, and now we know of hundreds, and we’re able to find smaller and smaller planets year by year. At this point in the science, educated guesses are helpful in generating hypotheses. If we learn that rocky planets of approximately Earth’s mass are common in the habitable zones of their solar systems, that will facilitate a huge shift in how we see ourselves in the universe. [ — Paragraph break — ] Kelly, thanks as always for explaining the science clearly and intelligibly.

  7. Tomasz Kokowski

    We all know, that there are many Earths out there in our Galaxy (at least). However, first – to find them we need to look inside the Galactic habitable zone – not to close too the its “hot” core and not too close to its “cold” fringes. This cuts number of search-worthy star systems by two-third I guess. Second – we, Earthlings seek planets resembling Earth as closely as it is possible. In fact we seek planets able to either harbor or with carbon cycle based life. Both terms: earth-like i terrestrial are the mind shortcuts only.

    Third – the way to find out is any Earth (anybody) out there is paved by refinement (evolution ?) of Our cosmic eyes. So, ten years ago We were able to detect Jupiter+ planets only. Five years ago we started to detect super-Earths (several mass of Earth) and planetary systems. Recently we spotted some first planets with few mass of Earth within habitable zones of their host stars. Refinement continues…

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