SETI Stars with planets seem to harbor "heavy" elements

Larry Klaes (
Fri, 30 Jul 1999 20:44:45 -0400

>From: Eugene Leitl <>
>Date: Fri, 30 Jul 1999 17:06:18 -0700 (PDT)
>To: <>
>Subject: tech: Stars with planets seem to harbor "heavy" elements
>X-Mailer: VM 6.71 under 21.1 (patch 4) "Arches" XEmacs Lucid
> Stars with planets seem to harbor "heavy" elements
>When astronomers first discovered planets around sunlike stars three
>and a half years ago, many cast their discoveries in a philosophical
>light. Earth and the rest of the sun's family, they affirmed, were
>just a few faces in the planetary crowd, not special at all and
>certainly not the center of the universe. "What we are seeing," Robert
>Brown of the Space Telescope Science Institute said at the time, "is
>the culmination of intellectual history that began with Copernicus 500
>years ago."
>With some 18 worlds definitively located--roughly one per 20 sunlike
>stars observed--astronomers now have enough planets to test that
>assertion. The findings have already undermined decades of
>conventional wisdom about what a planetary system should look like:
>half the planets orbit unexpectedly close to their stars; the other
>half have elongated orbits unlike any in our solar system. But less
>widely known is another mystery--unexplained patterns in the
>composition of the parent stars.
>In 1997 Guillermo Gonzalez of the University of Washington and his
>colleagues discovered that the first batch of these stars contained an
>unusually high concentration of most elements heavier than helium,
>known to astronomers as "metals." Of the 12 he has data on today, 10
>have an above-average metal content. Indeed, several are the most
>metal-rich stars in this area of the galaxy, with three times the
>endowment of the sun, itself enriched.
>Traditionally, astrophysicists have neglected the effect a planet
>could have on its star. But if the metal enhancement is related to the
>presence of planets, they will need to revisit both planet formation
>and stellar evolution. In one hypothesis, unless a star and its
>surrounding disk of dust and gas have a critical mass of
>metals--roughly equal to the amount in our solar system--planets can
>never coalesce. Not only do these elements make up rocky planets and
>the rocky cores of gas giant planets, they radiate heat more
>efficiently and thereby provide an essential cooling mechanism for the
>If planets need an extra dose of metals in order to form, they would
>be restricted to the inner reaches of the galaxy, where enough metals
>have been synthesized by successive generations of stars. Gonzalez
>also speculates that for a planetary system to support living things,
>it should not have too many metals, lest the worlds be continually
>bombarded by debris or tossed about by mutual interactions. Therefore,
>the galaxy may have a narrow "habitable zone" about halfway out the
>galactic disk, where the frequency of supernovae and stellar close
>encounters is also low. Of all the stars in the solar neighborhood,
>the sun traces the most nearly perfect orbit through this zone.
>Some astronomers, however, are doubtful. Douglas Lin of the University
>of California at Santa Cruz points out that the density of metals
>within our solar system varies a millionfold from the orbit of Mercury
>to that of Neptune. A slight overall metal enrichment would be lost
>among the internal variation that already exists.
>Lin and others have focused on an alternative hypothesis--namely, that
>the high metal content is an effect rather than a cause of planet
>formation. Planets nuzzled up to their stars must have moved inward
>from their original positions. Might some have spiraled all the way
>into their stars? Although it might seem unlikely that devouring a
>measly planet could affect the composition of an entire star, the
>metals from the ex-planet would be concentrated near the surface of a
>sunlike star. Downing a couple of Jupiters would make a noticeable
>difference in the observed metal content. Comets and asteroids, too,
>could provide the recommended allowance--smaller morsels to be sure,
>but plentiful.
>Imbibed bodies also bring angular momentum, which would have an
>especially strong effect on aged stars, causing them to spin faster
>and burp out gas. Mario Livio of the Space Telescope Science Institute
>estimates that roughly one in 20 senior stars shows signs of having
>digested a globe, a frequency comparable to the statistics of the
>planet hunters.
>Besides the metals trend, there is a tantalizing hint of another
>trademark of planet-bearing stars: a dearth of lithium. In 1997, when
>Geoffrey W. Marcy and R. Paul Butler of San Francisco State University
>and William Cochran and Artie Hatzes of the University of Texas at
>Austin discovered a planet orbiting one of the stars in the binary
>system 16 Cygni, they noticed that the star had much less lithium than
>its planetless twin. So far a lithium trend remains unsubstantiated.
>To pursue the mystery of what makes some stars fertile and leaves
>others barren, Cochran plans to look for planets in a sample
>specifically chosen to study the metals trend. Meanwhile investigators
>press forward on what is now their main goal: finding a full-fledged
>planetary system. Only such a system would quell--or
>vindicate--lingering doubts that these bodies really are planets,
>rather than very low mass brown dwarf stars (which form differently
>and should not be in such systems). In any event, it seems that the
>sun is not such a small, unregarded star at an unfashionable end of
>the galaxy after all. The ability to bear planets, let alone habitable
>ones, may not be universal.
> --George Musser
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