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The news has been filled with reports of exoplanets, planets orbiting other stars than our sun. These have been discovered by observing the dimming of the star due to transit -- the passing of the planet between the star and us.

That led me to consider whether an observer on a planet of another star could discover the existence of Earth by observing our transit. Only if the observer has very good instruments and is orbiting one of very-few stars. Consider:

The sun has a diameter of 864,000 miles; the Earth has a diameter of 7,900 miles; the average distance between the two -- the semi-major axis of  Earth's orbit -- is 92,900,000 miles. (Miles? this is from the 1958 edition of Van Nordstrom's Scientific Encyclopedia.) So the Earth will pass between any part of the photosphere and a star only if the star is within 0.53 degree of the ecliptic plane. It will intercept 0.00914 of the sun's radiation.

Mercury will transit for any observer circling a star within 1.38 degrees of it's orbital plane, but it has less than a tenth of the area, and thus will intercept less than a tenth of the light that Earth does. Jupiter will intercept 0.083 of the sun's light -- a significant occlusion -- but only for observers within a tenth of a degree of its orbital plane.

Now, of course, most of the planets discovered have been much closer to their primaries than Earth is to the Sun. Also, the planetary orbits in the Solar System are through planes somewhat near to the rotational plain of the galaxy.

Even so, there must be many more close stars with orbiting planets which we cannot observe simply because the line of observation is too far outside the plane of planetary orbit.

Originally posted to SciTech on Tue Feb 19, 2013 at 05:33 PM PST.

Also republished by Astro Kos.

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Comment Preferences

  •  That's very interesting (8+ / 0-)

    Another tidbit in the growing body of evidence that there are a lot more planets out there than we have thought so far.

    "The smartest man in the room is not always right." -Richard Holbrooke

    by Demi Moaned on Tue Feb 19, 2013 at 05:38:26 PM PST

  •  Stars without planets (5+ / 0-)

    are the exception, not the rule.

    Intelligent life?  Well, maybe somewhere . . .

    Fake Left, Drive Right . . . not my idea of a Democrat . . .

    by Deward Hastings on Tue Feb 19, 2013 at 05:46:34 PM PST

  •  For Non Transiting Planets We Can Sometimes (8+ / 0-)

    pick up the wobble in the star itself caused by the planet.

    I would guess this would be most visible for planets with orbital planes close to edge-on to us. Have we detected planet-induced wobble in any stars where the orbital axis is nearly pole-on toward us?

    We are called to speak for the weak, for the voiceless, for victims of our nation and for those it calls enemy.... --ML King "Beyond Vietnam"

    by Gooserock on Tue Feb 19, 2013 at 05:55:21 PM PST

    •  those are more difficult to detect (1+ / 0-)
      Recommended by:

      As wit transits, they require longer observational periods.

      I'm not sure if we could detect it at all in this way if it were "pole-on" to us, though.

      All things in the sky are pure to those who have no telescopes. – Charles Fort

      by subtropolis on Tue Feb 19, 2013 at 08:24:33 PM PST

      [ Parent ]

      •  We could. (0+ / 0-)

        A planet orbits not the star but the center af mass of the star-planet system. (Which is VERY close to the center of mass of the star; let's not fool ourselves.)

        Consider two systems which differ only in that one has us in the plane of the planet's orbit and the other has us on the line that is the pole of the planet's orbit. Then each planet would move as far from the center of gravity and back again as the other would. And so would each star.

        •  Erm?? (0+ / 0-)

          According to comments lower down, the motion detected is not motion perpendicular to the line between us and the star but motion along that line.

          In that case, orbiting in the plane at right angles to the line beween us and the star would be undetectable.

    •  We don't know. (0+ / 0-)

      When we use radial doppler shifts to detect planets, we don't get to know the actual orientation of the orbit. We only get a minimum mass of the planet, and the minimum is for an in-plane/eclipsing orbit. The further off (and closer to the vertical from our perspective), the more the actual planet mass would be.

      In cases where we get both an eclipse and a doppler shift, we can tell a bit more.

  •  Time to start sending interstellar probes (9+ / 0-)

    Yes, it will take 100 to 200 years to get to Alpha Centari and Tau Ceti, and somewhat longer to get to Gliese. There's a huge amount we can learn by getting a closer look at these planetary systems, so it's worth the long wait. Send out a probe every four years or so, and a few of them will reach their destination.

    All that's remaining is to make sure we're still capable of processing the data the probes send back.

    •  Time to develop Bussards Polywell based drive (3+ / 0-)
      Recommended by:
      antirove, subtropolis, mookins

      Well if Polywell works, which we dont yet. 37 days to Mars, 76 to Titan.

      555 AU in 5 years, to leave a telescope that uses the Earth as a grav lense.

      FDR 9-23-33, "If we cannot do this one way, we will do it another way. But do it we will.

      by Roger Fox on Tue Feb 19, 2013 at 07:32:19 PM PST

      [ Parent ]

      •  37 days to Mars, maybe (2+ / 0-)
        Recommended by:
        mookins, Roger Fox

        But when you start talking inter-stellar travel, we don't know yet just how fast a spacecraft can go and still hold together. Yes, there's no gravity or friction in inter-stellar space, but there's still dust and gas. For the time being, we need to be conservative and measure travel time in hundreds of years.

        And the sooner we get started, the better.

      •  Subjective... (0+ / 0-)

        for someone on the probe. For those of us back here, it will still take well over four years to get to Alpha Centauri, and four more years to get data back. I'd actually be very surprised if we could get a probe there in 12 years, even with a working, efficient fusion engine, plus four more years for data return by laser.

  •  I believe the wobble is a better clue (1+ / 0-)
    Recommended by:

    That is the motion induced upon a star by its orbiting companions (children?) can be detected via tiny variations in its emitted wavelength (spectral lines). As i understand it, this type of observation isn't as sensitive to divergence from that star's ecliptic.

    I think.

    Anyway, your point stands. Astronomers have been rapidly increasing the rate at which they can identify potential exoplanets. It stands to reason, then, that an intelligent civilisation could have pinpointed our own planet many eons ago. They may even have concluded that life existed here without leaving their home system.

    Kinda puts the cork in the "Why would aliens bother visiting this backwater?" argument. After all, it shouldn't betoo long before we can spot not just habitable planets, but ones that almost certainly harbour life. Would we ignore it? Most assuredly not.

    All things in the sky are pure to those who have no telescopes. – Charles Fort

    by subtropolis on Tue Feb 19, 2013 at 08:19:22 PM PST

  •  I heard something about exoplanets on public radio (1+ / 0-)
    Recommended by:

    It was an interview with a woman who’s working on the Kepler project. They’re discovering hundreds of planets, of which many are Earth-sized. Whether they are earth-like is another question. Two quick comments:

    I think there might be life of some sort on some of the planets but I doubt we’ll ever visit them (in person) or that they will visit us. I think the speed of light is a firm limit and it would be difficult to get anywhere without a voyage of decades or maybe centuries. I suppose we could send some kind of unmanned probe, just to see what’s there, but even that seems unlikely.

    Second thought: I wonder about religion. If there are other planets and there’s life there, do they have religious beliefs? I don’t necessarily believe that Jesus died for my sins, but if he did, did he then go to other planets and die over and over again?

    “If you misspell some words, it’s not plagiarism.” – Some Writer

    by Dbug on Tue Feb 19, 2013 at 09:08:00 PM PST

  •  I guess this goes to show that detecting a (0+ / 0-)

    planet solely based on zeroth order geometrical optics is extremely limited.  It doesn't mean the planets aligned to some other ecliptic plane are undetectable; it just means we need to use more sophisticated techniques and more sensitive instruments.

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