Astronomy Without A Telescope – Planet Spotting

Article written: 8 May , 2011
Updated: 24 Dec , 2015
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The Extrasolar Planets Encyclopedia counted 548 confirmed extrasolar planets at 6 May 2011, while the NASA Star and Exoplanet Database (updated weekly) was today reporting 535. These are confirmed findings and the counts will significantly increase as more candidate exoplanets are assessed. For example, there were the 1,235 candidates announced by the Kepler mission in February, including 54 that may be in a habitable zone.

So what techniques are brought to bear to come up with these findings?

Pulsar timing – A pulsar is a neutron star with a polar jet roughly aligned with Earth. As the star spins and a jet comes into the line of sight of Earth, we detect an extremely regular pulse of light. Indeed, it is so regular that a slight wobble in the star’s motion, due to it possessing planets, is detectable.

The first extrasolar planets (i.e. exoplanets) were found in this way, actually three of them, around the pulsar PSR B1257+12 in 1992. Of course, this technique is only useful for finding planets around pulsars, none of which could be considered habitable – at least by current definitions – and, in all, only 4 such pulsar planets have been confirmed to date.

To look for planets around main sequence stars, we have…

The radial velocity method – This is similar in principle to detection via pulsar timing anomalies, where a planet or planets shift their star back and forth as they orbit, causing tiny changes in the star’s velocity relative to the Earth. These changes are generally measured as shifts in a star’s spectral lines, detectable via Doppler spectrometry, although detection through astrometry (direct detection of minute shifts in a star’s position in the sky) is also possible.

To date, the radial velocity method has been the most productive method for exoplanet detection (finding 500 of the 548), although it most frequently picks up massive planets in close stellar orbits (i.e. hot Jupiters) – and as a consequence these planets are over-represented in the current confirmed exoplanet population. Also, in isolation, the method is only effective up to about 160 light years from Earth – and only gives you the minimum mass, not the size, of the exoplanet.

To determine a planet’s size, you can use…

The transit method – The transit method is effective at both detecting exoplanets and determining their diameter – although it has a high rate of false positives. A star with a transiting planet, which partially blocks its light, is by definition a variable star. However, there are many different reasons why a star may be variable – many of which do not involve a transiting planet.

For this reason, the radial velocity method is often used to confirm a transit method finding. Thus, although 128 planets are attributed to the transit method – these are also part of the 500 counted for the radial velocity method. The radial velocity method gives you the planet’s mass – and the transit method gives you its size (diameter) – and with both these measures you can get the planet’s density. The planet’s orbital period (by either method) also gives you the distance of the exoplanet from its star, by Kepler’s (that is Johannes’) Third Law. And this is how we can determine whether a planet is in a star’s habitable zone.

It is also possible, from consideration of tiny variations in transit periodicity (i.e regularity) and the duration of transit, to identify additional smaller planets (in fact 8 have been found via this method, or 12 if you include pulsar timing detections). With increased sensitivity in the future, it may also be possible to identify exomoons in this way.

The transit method can also allow a spectroscopic analysis of a planet’s atmosphere. So, a key goal here is to find an Earth analogue in a habitable zone, then examine its atmosphere and monitor its electromagnetic broadcasts – in other words, scan for life signs.

Direct imaging of exoplanet Beta Pictoris b - assisted by nulling interferometry which removes Beta Pictoris' starlight from the image. The red flares are a circumstellar debris disk heated by the star. Credit: ESO.

To find planets in wider orbits, you could try…

Direct imaging – This is challenging since a planet is a faint light source near a very bright light source (the star). Nonetheless, 24 have been found this way so far. Nulling interferometry, where the starlight from two observations is effectively cancelled out through destructive interference, is an effective way to detect any fainter light sources normally hidden by the star’s light.

Gravitational lensing – A star can create a narrow gravitational lens and hence magnify a distant light source – and if a planet around that star is in just the right position to slightly skew this lensing effect, it can make its presence known. Such an event is relatively rare – and then has to be confirmed through repeated observations. Nonetheless, this method has detected 12 so far, which include smaller planets in wide orbits such as OGLE-2005-BLG-390Lb.

These current techniques are not expected to deliver a complete census of all planets within current observational boundaries, but do offer us an impression of how many there may be out there. It has been speculatively estimated from the scant data available so far, that there may be 50 billion planets within our galaxy. However, a number of definitional issues remain to be fully thought through, such as where you draw the line between a planet versus a brown dwarf. The Extrasolar Planets Encyclopedia currently set the limit at 20 Jupiter masses.

Anyhow, 548 confirmed exoplanets for only 19 years of planet spotting is not bad going. And the search continues.

Further reading:
The Extrasolar Planets Encyclopedia
The NASA Star and Exoplanet Database (NStED)
Methods of detecting extrasolar planets
The Kepler mission.

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19 Responses

  1. mrbill says

    Truly a historic announcement.

  2. TerryG says

    Was hoping the Astrometry method would finally have detected some wide orbit exoplanets until the Space Interferometry Mission (SIM Lite) was canceled. The over budget and behind schedule James Webb Telescope seems to be sucking the $$$ out of other projects. Thanks Northrup Grumman.

  3. Moonshine says

    The James Webb Telescope better be worth the money spent on it. If not,trash it,and lockout Northrup Grummam from doing anymore jobs for the Federal Government. Perminatly!!!
    I have NO patience for excuses for delayed launches and cost over runs. That might get the attention too the other leeches on the Tax Payer’s dime!
    It’s time to dishout the penalties.
    Kepler to me has been a great mission,and shows the greatness of the people behind the scene. Go K !!!!

  4. Member

    Thanks IVAN3MAN_AT_LARGE 🙂

  5. Pradipta says

    Nothing is real beyond 150 million kilometers. Milky ways, galaxies, supernovas, white dwarfs, nebulas, black holes, extra solar systems etc. are all antic views. To know the fact visit: http://www.spacemirrormystery.com

    • Mr Mike says

      @ Pradipta

      Please take your kooky pet theories away from here, do not return until you can back your fraudulent claims with facts which are provable and repeatable by other scientists.

      In this theory you posit too many un-facts, non-facts, and bald lies; put some hair on those at least.

      I realize english is not your mother tongue but please make the effort to learn its usage as a tool in your attempts at communication. The laughable “antic views” of your devising above contribute much to your humorous undertaking.

      Mr Mike

    • Uncle Fred says

      Pradipta, this is a website dedicated to factual research, not whimsical fiction.

      There are no space mirrors. Voyager 1 is 17.4 billion kilometers from Earth and counting.

      http://voyager.jpl.nasa.gov/mission/weekly-reports/index.htm

      This fact alone destroys your theories. You can dispute this value, but you better be damned sure your objection is backed by math! Good luck on this one, the same physics that NASA used to calculate Voyager’s distance to the Earth has allowed the agency to land of planets, moons, and explore first hand numerous cosmic phenomenon.

      You also sell your theories substantial sums of money. It is likely, before you ever get a chance to reply, your comments will be removed from this site.

      Good luck.

  6. microverses says

    Good stuff! You know – I’d like to see some time spent looking towards the Orion area as there’s countless tales of visits to earth from that area in space.

    • Torbjorn Larsson OM says

      You know, for “countless tales” they are pretty anonymous. You could have lest mentioned _one_!?

      • Jon Hanford says

        “You know, for “countless tales” they are pretty anonymous. You could have lest mentioned _one_!?”

        H.P. Lovecraft

        ‘Beyond the Wall of Sleep’ :

        “Of the oppressor I cannot speak. You on earth have unwittingly felt its distant presence – you who without knowing idly gave the blinking beacon the name of Algol, the Demon-Star It is to meet and conquer the oppressor that I have vainly striven for eons, held back by bodily encumbrances. Tonight I go as a Nemesis bearing just and blazingly cataclysmic vengeance. Watch me in the sky close by the Demon-Star.

        “I cannot speak longer, for the body of Joe Slater grows cold and rigid, and the coarse brains are ceasing to vibrate as I wish. You have been my only friend on this planet – the only soul to sense and seek for me within the repellent form which lies on this couch. We shall meet again – perhaps in the shining mists of Orion’s Sword, perhaps on a bleak plateau in prehistoric Asia, perhaps in unremembered dreams tonight, perhaps in some other form an eon hence, when the solar system shall have been swept away.”

      • Torbjorn Larsson OM says

        LOL!

      • microverses says

        Zulu Nation – the chitihoori is one tale. There are even crude, yet correct, instruments that depict the complete system from thousands of years ago – showing Sirius B despite it not being visible by the naked eye.

        The annuki, the greys….all of these have tales that take them back to Orion region.

      • Torbjorn Larsson OM says

        Good. Now supply the evidence backing that tale.

        (Tip: You can’t, it’s a religious tale of Zulu shamans.)

      • microverses says

        Soooooo…….. you’re busting my balls for what purpose then?

        This is part of the ancient tales of creation and mankind – neither of us were around and obviously we’ve made a few runs around the sun since then. I’m not making the stuff up – it’s just that I am aware of these tales. I did not say I say subscribe to them, did I? Nope. Didn’t do it.

        I thought this was about exploration and discovery – not censoring out thought patterns to keep hold of a static image of the whats and whys.

        It seems to me a ‘why not’ scenario. It comes up over and over.
        I say – there are tales, we have some technology capable – why not take a look? It’s not like we’re going to find a bustling planet full of technologically advanced life. Jeez.

      • Jon Hanford says

        “Zulu Nation – the chitihoori is one tale. There are even crude, yet correct, instruments that depict the complete system from thousands of years ago – showing Sirius B despite it not being visible by the naked eye.”

        This sounds similar to claims made by the Dogon tribe in Africa. Carl Sagan, Isaac Asimov and Ian Ridpath have all been critical of the Dogon myths wrt Sirius B, especially Robert Temple’s massive “unreadable[Asimov]” tome on the subject.

        Noted author James Oberg has a detailed account of “The Sirius Mystery”: http://www.debunker.com/texts/dogon.html

        (My earlier Lovecraft quote refers to the Cthulhu Mythos used in several of his short stories. The ‘Ancient Ones’ were described as beings originating from the Orion region of the sky)

      • Lawrence B. Crowell says

        Hmmm, I have not heard the Metallica song “Call of Ktulhu” recently. H.P. Lovecraft created a sort of alt-cosmology where the universe is guided by entities that have a pitiless regard for our concerns. There is a description of the hosts which dance around Cthulhu in some process which maintains the order of things, while Cthulhu’s avatar sleeps in the ocean depths. Replace Cthulhu with black holes that can quantum mechanically tunnel out “anything,” and with other science we have indeed learned we live in an utterly pitiless reality.

        Curiously there are some Christian fundamentalist trends which have the Orion Nebula as the cosmic sanctuary of Jesus Christ. The Jehovah Witnesses and Seventh Day Adventists hold to this idea, where the nebula is what the sanctuary of Mosaic Law as spelled out in Exodus and Deuteronomy is supposed to be modeled on. Years ago I got into a discussion with somebody who held this idea, where I mentioned how other galaxies, such as the Whirlpool galaxy, have rosette displays of such nebula and the Orion Nebula is one of dozens in our own galactic neighborhood. The Orion Nebula is only exceptional in that a modest telescope can bring it to view. It is just close up.

        BTW, there is a guy named Camping who is telling us the Rapture is coming May 18 or 20. Cool! And he is selling tickets, “send in money so I can get this message out.” Maybe Jesus will come flying in from the Orion Nebula. Hell of a damned show that would make!

        LC

      • microverses says

        Thanks, I’ll print that out to read later. I remember seeing the tool – and it was Sumerian now that I think back.

        Lovecraft was one to make many references to stars, or ‘unblinking eyes’ in the sky. One of my favs is The Hound.

  7. Torbjorn Larsson OM says

    Thanks for the run through (“space out”?)!

    To think that a few decades ago they thought we would _never_ detect other planets. Now we can direct image them, map them, even search for life signs. My 2 $s: puts the A in Astronomy.

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