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Planetary Systems

Extrasolar Planets

Another World Ancient World In the summer of 2003 astronomers have discovered a planetary system more similar to our own Solar System than any known previously. The bright star HD70642, visible with binoculars toward the constellation of Puppis about 90 light years away, was already known to be a star like our Sun. Now a planet with twice Jupiter's mass has been discovered in a nearly circular orbit at approximately half the orbital distance of Jupiter. Such an orbit allows the possibility of habitable Earth-type planets orbiting further in, a possibility not likely with all previously discovered planetary systems with massive planets

Figure 07-26 Another World [view large image]

Figure 07-27 Ancient World [view large image]

occupying disruptive elliptical orbits. Figure 07-26 indicates what the HD70642 planetary system might look like from a hypothetical moon orbiting the newly discovered planet.

Meanwhile it is discovered that a planet, a white dwarf, and a neutron star orbit each other in the giant globular star cluster M4, some 5,600 light-years away. The most visible member of the trio is the white dwarf star, indicated in an image from the Hubble Space Telescope (see Figure 07-27), while the neutron star is detected at radio frequencies as a pulsar. A third body was known to be present in the pulsar/white dwarf system and a detailed analysis of the Hubble data has indicated it is indeed a planet with about 2.5 times the mass of Jupiter. In such a system, the planet is likely to be about 13 billion years old. Compared to our solar system's tender 4.5 billion years and other identified planets of nearby stars, this truly ancient world is by far the oldest planet known, almost as old as the Universe itself. Its discovery as part of an evolved cosmic trio suggests that planet formation spans the age of the Universe and that this newly discovered planet is likely only one of many formed in the crowded environs of globular star clusters.

The red object in Figure 07-28a shows the first image of a possible extrasolar planet taken by an infrared camera at the ESO's Paranal Observatory in Chile. The possible planet is about five times as massive as Jupiter. It revolves around a brown dwarf (with a mass about 42 times less than the mass of the Sun) in an orbit roughly twice the Earth-to-Neptune distance. The object is still contracting into its final form and so is very warm, at 1,000oC. This system is 230 light-years away. In 2005, astronomers confirmed that a 5-Jupiter-mass gas-giant planet (2M1207b) is traveling through space with the brown dwarf. Direct-imaging is possible for this planet because the brown dwarfs evolve toward
Extrasolar Planet 1 Extrasolar Planet 2 cool surface temperatures (less than 700oC) and low luminosities. Meanwhile the infrared image of another extrasolar system has been captured in 2008 by the Gemini Observatory on Mauna Kea, Hawaii (Figure 07-28b). The parent star for this planetary system is similar in mass but a little cooler than the Sun because it is much younger (only a few million years old). Spectra data reveal the nature of the companion planet, which has a mass about 8 times that of Jupiter, and lies roughly 330 times the Earth-Sun distance from its star (about 10 times the distance from the Sun to Neptune). It is still red hot (1500oC) with heat generated during its formation by gravitational

Figure 07-28a Extra- solar Planet 1

Figure 07-28b Extrasolar Planet 2

contraction (see little red object at 11 o'clock). This system is 500 light-years away toward the constellation Scorpius.

Extrasolar Planet Detection At the end of 2007, the number of discovered extrasolar planets has increased to 270. The number is going up week by week. The detection methods (Figure 07-29a) now include:

  • The Doppler effect as the star wobbling toward us and away - This is the leading method accounting for 90% of the discoveries. However, it yields only a minimum mass unless the planet's orbital inclination can be determined by some other mean.
  • The dimming of star light by the transit of planet in front of the star - Such method can yield very accurate mass estimate. But only a small fraction of all planets transit their parent star's disk along our line of sight.
  • Gravitational microlensing of the star light by the planet - This method is capable of finding Earth-mass planets, although the chance of such alignment is rather rare.
  • Direct-imaging - This technique requires taking a planet's image (see Figure 07-28). It is a difficult task because of the glare of the star.
  • Figure 07-29a Methods of Detection

  • Einstein's Beaming - This is an effect that causes the star to brighten as it moves toward us, and dim as it moves away. The brightening is the result from photons "pile-up" (in energy). This technique requires measurements of stellar brightnesses with accuracy to few parts per million.
  • Einstein's Beaming As of 2013, such high-quality measurements has been achieved in "Center for Astrophysics" (CfA). By combining the Einstein's Beaming with tidal distortion of the central star, they have discovered Kepler-76b (also known as Einstein's planet, Figure 07-29b), which is a "hot Jupiter" that orbits its star every 1.5 days. Its diameter is about 25% larger than Jupiter, and weights twice as much. The central star is Type F located 2,000 light years from Earth in the constellation Cygnus. The discovery has since been confirmed by closer examination of the Kepler data, and by other observations.

    Figure 07-29b Einstein's Beaming

    Considerable excitement was generated in the news media on November 13, 2008 with the direct imaging of planets in extrasolar systems (now called exoplanet). One of them is circling a bright, young, star (Fomalhaut), about short 25 light-years away. Its mass is about 3 times the mass of Jupiter, and it moves in an orbit 10.7 billion miles from the star (almost 14 times the Sun-Jupiter distance). It is shown as a little red dot
    Exoplanet 1 Exoplanet 2 in Figure 07-30a with the camera's coronagraph to reduce the over-whelming glare from the star. The other direct image in Figure 07-30b shows three exoplanets (b, c, d red dots) each 5-13 times the mass of Jupiter, in orbit around HR 8799, a star 130 light years from Earth. The arrows in the image indicate the direction and magnitude of the planet's velocity. Actually, these claims have yet to be examined further to see if they are really exoplanets or brown dwarfs, which are failed stars just over 13 Jupiter masses.

    Figure 07-30a Exoplanet Fomalhaut b

    Figure 07-30b Exoplanets
    HR 8799b, c, d

    HARPS The discovery of 32 exoplanets was announced at the European Southern Observatory/Center for Astrophysics in October 2009. The detections were made possible by using the tactic of HARPS (High Accuracy Radial Velocity Planet Searcher in the La Silla Observatory, (see Figure 07-31), which can detect slight wobbles of stars as they respond to tugs from exoplanets' gravity. The instrument can measure movements as small as 1 m/s (2 mph) - a slow walking pace. With the discovery, the tally of new exoplanets found by HARPS is now at 75, out of about 400 known exoplanets.

    Figure 07-31 HARPS in La Silla Observatory [view large image]

    Kepler Space Telescope Detected Exoplanets The Kepler space telescope (Figure 07-32a) was launched on March 6, 2009. Its mission is to estimate the fraction of Sun-like stars that have Earth-like planets. It is using the "transits" method, which is most suitable for finding earth-like planets as mentioned earlier. Kepler will search 100000 pre-selected Sun-like stars 180-920 parsecs away (toward the direction of the Cygnus and Lyra constellations), sending back data to Earth every 30 days. Of the 342 exoplanets detected to date (March 2009), only 58 transiting planets have been found as shown in Figure 07-32b. Most of the exoplanets have been detected through the radial velocity method.

    Figure 07-32a Kepler Space Telescope [view large image]

    Figure 07-32b Detected Exopanets

    Kepler Planets In the beginning Kepler was bias in detection of giant planets with fast orbits that were too close to their host stars to be habitable (Diagram b*, Figure 07-33a). But by the end of 2010, more Earth-like planets were discovered including the Kepler-11, in which as many as 6 different planetary transits have been observed (see Diagram a for an artist's rendition). Among these planets, Kepler-11b is the most Earth-like with 4 times the mass and 2 times the radius of the Earth. Its density of 3 gm/cm3 implies unambiguously rocky

    Figure 07-33a The Kepler Planets [view large image]

    composition (see Table 07-01). However, its closeness to the star (~ 0.1 au) means that one side will be constantly molten rock. Because of the time needed for repeat observations of planets in Earth-length orbits, it will be years before Kepler researchers can establish the frequency of planets in the cosmos.
    *Kepler-11g has mass < 300 MEarth and orbital radius ~ 0.46 au.

    Kepler-20 The February 9 2012 issue of Nature reports the discovery of two Earth-sized exoplanets revolving around a Sun-like star called Kepler-20. They are designated as Kepler-20e and Kepler-20f (Figure 07-33b). The detection of Earth-sized planets is difficult because it is not very sensitive to the Doppler technique and the transition method can be mistaken by another star in a binary system. It is only through a combination of complementary measurements and statistical analysis that the detection is secured. Table 07-05 below lists some properties for the two Earth-sized planets. Data for the radius and semi-major axis were derived from

    Figure 07-33b Kepler-20
    [view large image]

    the Kepler photometry (producing the transit light curve). Planetary masses were estimated via theoretical considerations assuming that these are rocky bodies (Figure 07-33a). However, if we take the data for the
    radius and mass literally the density of the planets can barely qualified as rocky even at the upper range of the estimated mass of 3 MEarth, (see Table 07-01 about the range of density for rocky planets). Equilibrium temperature is defined by equating the input energy from the star to the output energy of the planet via reflection (known as albedo, it is assumed to be 0.6 for these cases). Anyway, it is too high for the planets to retain any liquid water. These planets may be Earth-sized but definitely not habitable. They are too closed to the parent star.

    Property Kepler-20e Kepler-20f
    Radius (REarth) 0.868 1.03
    Semi-major Axis (au) 0.05 0.1
    Mass (MEarth) 0.39 - 1.67 0.66 - 3.04
    Density (Density of Water) 0.3 - 1.25 0.5 - 2.28
    Equilibrium Temperature (oK) 1040 705

    Table 07-05 Properties of Kepler-20e and Kepler-20f

    Census by  Kepler On February 2, 2011 the Kepler team provided an estimate of 34% of the stars (150,000 in total so far) that host a planet with period less than 125 days (covered by 4 months of observations). This is Fp, the fraction of stars with planets in the Drake Equation. The team also suggest that 17% of these systems contain more than one planet. This figure is more controversial because the Kepler telescope is not able to detect those planets outside the edge-on view. Actually, models of planetary systems predict fewer single-

    Figure 07-34 Census by Kepler
    [view large image]

    planet systems than that from observations. Figure 07-34 is a census of size among some of the planets observed by Kepler. Those in habitable zone are circled in black.

    By the spring of 2012, NASA's Kepler mission has discovered 2321 exoplanet candidates with the method of stellar dimming, which can reveal the planet's size (Figure 07-35a). In order to qualify for the status of confirmed planet, members on the list have to be verified by another method, namely the detection of stellar wobbling, which can determine a planet's mass. This function can be performed by HARPS located at the La Silla Observatory in Chile. The problem is that Kepler looks north so that the validation can be done only marginally through one of the twin 10-metre telescopes at the Keck Observatory atop Mauna Kea in Hawaii.
    Kepler Canidates, 2012 Kepler Candidates, 2013 Kepler, Earth-like Exoplanets The logjam would be removed on 1 April, 2012 with the establishment of HARPS-North at the 3.6-metre National Galileo Telescope on La Palma in the Canary Islands.

    Figure 07-35a Kepler Candidates, 2012 [view large image]

    Figure 07-35b Kepler Candidates, 2013

    Figure 07-36a Kepler, Earth-like Exoplanets [view large image]

    The project would also facilitate the discovery of Earth-like planets since
    the combined data would show both the size and mass. Figure 07-35b shows the increase in the number of candidates discovered by Kepler (comparing to the 2011 data) as of January, 2013. NASA announced on April 2013 the discovery of three super Earth size exoplanets in habitable zone by Kepler (Figure 07-36a). Kepler-62f is potentially the most Earth like exoplanet out of the new discoveries. It could be rocky, with polar caps, land mass and water as well. It goes around its star once every 267.3 (Earth) days.

    It was reported on May 2013 that the Kepler telescope has turned into "self-protective" and "safe" modes. It means that the system is in serious trouble as two of its four spinning wheels (for stabilizing the telescope) fail to work. Recovery effect is underway as of August 2, 2013 (see "Kepler Mission Manager Update").

    More Earth-likePlanets Meanwhile, search for Earth-like planets is on-going by repeatedly trolling or sifting through the data already collected (in the previous 4 years) - it is something like looking for a piece of jewel in a garbage dump. Eventually, additional two more (Kepler -442b and -438b) were found in 2014, another one (Kepler-186f) in January, 2015 (see Figure 07-36b).

    Figure 07-36b More Earth-like Planets [view large image]

    Kepler-452b The Kepler-452b was announced in July, 2015 with considerable fanfare - that it is the most Earth-like (Figure 07-36c and 36b). This planet is 5 times as massive as Earth putting it in the category of rocky planets. Its parent star belongs to the G class in the HR diagram (similar to the Sun) but about 1.5 billion years older. Therefore, we are witnessing Earth's future by looking at this system. In addition to be located in the habitable zone, Kepler 452b whirls around its star every 385 days - very close to our year of 365 days.

    Despite two disabled spinning wheels, by 2015 Kepler has been revived to operate in "K2" mode, which limits the observation to the ecliptic plane.

    Figure 07-36c Kepler-452b [view large image]

    Rogue Planets Two international teams of astronomers announced in 2011 that unbound planets may be very common - probably hundreds of billions of these rogue planets (planets without a host star) move around the Milky Way un-detected so far. The detections utilized the micro-lensing technique, which increases the brightness of a background object as an unbound planet passes in front of it (Figure 07-37). Two detectors together (such as a telescope on Earth and the Spitzer Space Telescope) can determine the distance to the planet as well. A rogue planet probably entered interstellar space when gravitational interactions with other celestial bodies flung it from its parent star. It seems that planets are easier to make than to keep.

    Figure 07-37 Rogue Planets [view large image]

    Simple calculation shows that even there are about 1011 galaxies in the universe, the amount of rogue planets is 6 order of magnitude short of the total amount of dark matter (in mass).

    Figure 07-38 presents the artist's renditions for some of the extrasolar planets:
    Extrasolar Planets 1. HD 168443 - Tidal forces could melt ice on a possible moon of this massive planet.
    2. HD 16141 - A possible moon's arid landscape attests to this gas giant's proximity to
         its Sun-like star.
    3. HD 209458b - This jupiter-like planet is so close to its parent star that its heated      atmosphere (including water vapor) is simply expanding away into space.
    4. PSR B1257+12 - The first exoplanets discovered, these three worlds orbit an

    Figure 07-38 The Other Worlds [view large image]

         energetic pulsar.
    5. HD 189733b - This is the first picture of an exoplanet obtained by subtracting light
         from its parent star. It is a giant planet similar to Jupiter.
    6. 581 c - The most earth-like planet discovered yet (2007) beyond the solar system is in the remarkable system of three planets orbiting the red dwarf star Gliese 581, which is 20 light-years away toward the constellation Libra. One of the planets 581 c is the smallest planet orbiting the star. It is estimated to be five times as massive as Earth with about 1.5 times Earth's diameter. That super-earth orbits once every 13 days, about 14 times closer to its parent star than the Earth-Sun distance. The close-in orbit around the cool star implies a mean surface temperature of between 0 and 40 degrees C - a range over which water would be liquid - and places the planet in the red dwarf's habitable zone.

    See "Worlds of Pure Imagination" by NewScientist, September 2015.

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