Twin Planets Survive Solar Blow-Out

A team of astronomers have discovered two new planets orbiting giant stars, bringing to 118 the total number of known extrasolar planets. At least seven substellar companions have been found orbiting K-type stars, providing evidence that a planet can survive the evolution of its host star into a giant.

kgiant
Sloan Digitized Sky Survey image of the star HD 59686. Credit: SDSS/PlanetQuest

Several billion years from now, our Sun will evolve into a giant star similar to those the team has observed. The Earth will receive about 60 times more radiation than it does today and the temperature will rise to several hundred degrees Celsius. Observing the fate of companions of other stars late in life provides a glimpse into the future of our own solar system.

The doppler effect
Aging star encroaches on orbits of inner planets
Credit: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics

In a report at the American Astronomical Society meeting in Atlanta, Georgia, the team described four new companions to giant stars, two of which are of brown dwarf mass. One of the planetary candidates orbits the star 91 Aquarius, 146 light-years away [92 parsecs].

It has a minimum mass about three times that of Jupiter and orbits its host star once every 182 days. The other planet orbits the star HD 59686 once every 303 days and has a minimum mass of six-and-one-half Jupiters.

Located 299 light-years away, the star has a magnitude of 5.5 and should be visible to the unaided eye under favorable circumstances. K-type stars are sometimes referred to as ‘orange stars’, because they represent a class of relatively cool stars with surface temperatures in the range of 3,500 to 5,000 degrees Kelvin, a typical spectral region for orange coloration.

The discoveries were made with the Doppler technique, where the gravitational pull of the planet causes a wobble in the measured velocity of the host star.

Geoff Marcy of Berkeley described the challenges of finding such new planets: "The discovery of planets around other stars is extraordinarily difficult. The problem, in a nutshell, is that a planet is about 1 billion times fainter than its host star, so the planets get lost in the glare of the host star. The reason we’re finding planets now, by this doppler technique, is that now we have big telescopes, fast computers and most importantly, exquisite optics."

The observations were carried out with the Hamilton echelle spectrograph at the University of California’s Lick Observatory as part of a survey of 182 K-giant stars. Observations of HD 59686 began in June 1999, and the other three stars were observed beginning in June 2000, with observations continuing to the present.

Dust to Dust

Finding a giant star with surviving planets is only part of whether any life on that planet could survive the inevitable expansion of its host sun.

mars_colony
"The luminosity of the sun-like stars changes very gradually, over millions of years. This is enough time to mount a massive technological program to move outwards in the planetary system." -Frank Drake
Image Credit: AnimaTek Int.

The lifecycle of such burnt-out stars, as they age toward a giant phase, has been considered by paleogeologist, Peter Ward, author of the book, Rare Earth: "Our Sun has about another 7 billion years before it enters the Red Giant phase. Surely, then, we could expect a long period of habitability. But the reality is that it takes more than the correct amount of solar energy to make a planet habitable."

"This is especially true for complex organisms such as animals," said Ward, "which have a very narrow range of temperatures and nutrient requirements compared to microbes. The presence of complex life on the Earth will end in no more than a billion years (and perhaps much sooner), due to a sequentially predictable breakdown of habitable systems on our planet. The systems in question are those that serve to regulate the Earth’s temperature and atmospheric carbon dioxide content…The period of time that one can expect complex life to exist will vary from world to world. Our ‘Rare Earth’ hypothesis is that on most planets, this will be too short a time to allow complexity to arise at all. "

"For our own star", Ward concluded, "the flaring into a red giant will be followed by a stellar retreat into a dwarf stage that will last untold billions of years. As astronomers gaze out into the heavens with their powerful telescopes, they see billions of such stellar tombstones. The galaxy is littered with dead stars, the markers of how many dead planets, and of how many dead civilizations that for a time circled these stars when they were young and vigorous."

Frank Drake, of the SETI Institute, considers that advanced technologies may even counteract something as catastrophic for a civilization as losing its energy source. In addition to a civilization just migrating to outer planets far enough away from their expanding star, Drake imagines ways to put such an aged sun onto emergency life support: "Once a species has developed high technology, there are many strategies for dealing with the changing brightness of the home star. It has even been suggested by Gregory Benford that the main sequence lifetime of stars can be greatly extended by developing a technology which stirs the star, bringing fresh hydrogen to the core – after all, about 90% of a star’s mass is intact when the giant stage is approached. A far-out idea to be sure, but it reminds us that clever technologies may be as yet unrecognized by us."

What’s Next

Scientists hope to launch six new space-borne missions over the next few years to search for terrestrial planets.

They include France’s small-scale COROT, NASA’s Kepler mission and Space Interferometry Mission (SIM).

The French COROT mission, approved and due for launch in late 2004, will study asteroseismology, or oscillations within stars, and likely will be the first orbiting telescope to search for extrasolar planets.It will look at 50,000 to 60,000 stars and should find a few dozen terrestrial planets and several hundred close-in gas-giant planets during a two- to three-year mission, says Pierre Barge, an astronomer at the Laboratory of Astrophysics in Marseille and leader of COROT’s exoplanets group. COROT – for Convection, Rotation and Planetary Transits – is a mission of CNES, the French National Center for Space Studies, in partnership with ESA, Italy, Belgium and Germany. When searching for extrasolar planets, COROT’s 27-centimeter (10.6-inch) telescope will use a method called photometry, in which sensitive light detectors look for a slight drop in a star’s brightness as a small planet "transits" the star (crosses the face of, or eclipses, the star as viewed from COROT).

SIM reference design
SIM, scheduled for launch in 2009, will determine the positions and distances of stars several hundred times more accurately than any previous program.
Credit: NASA / JPL

The Kepler mission is scheduled for launch into solar orbit in October 2006. Kepler will simultaneously observe 100,000 stars in our galactic "neighborhood," looking for Earth-sized or larger planets within the "habitable zone" around each star – the not-too-hot, not-too-cold zone where liquid water might exist on a planet. To highlight the difficulty of detecting an Earth-sized planet orbiting a distant star, it would take 10,000 Earths to cover the Sun’s disk.

marcy
Geoff Marcy, a planet hunter from UC Berkeley
Image Credit: astron.berkeley.edu

In space, the Kepler Mission can achieve 100 times better photometric precision than Earth-based telescopes looking through the atmosphere, and this will allow the team to search for much smaller planets, like Earth. The Earth is 100 times smaller in area than Jupiter. Kepler has the opportunity to discover and confirm through detection of repeated transits that there are Earth-size planets in Earth-like orbits around other suns.

One NASA estimate says Kepler should discover 50 terrestrial planets if most of those found are about Earth’s size, 185 planets if most are 30 percent larger than Earth and 640 if most are 2.2 times Earth’s size. In addition, Kepler is expected to find almost 900 giant planets close to their stars and about 30 giants orbiting at Jupiter-like distances from their parent stars. Because most of the gas giant planets found so far orbit much closer to their stars than Jupiter does to the Sun, during the four- to six-year mission, Kepler will find a large proportion of planets quite close to stars. If that proves true, scientists expect to find thousands of planets.

Due for launch in 2009 is the almost $1 billion NASA-ESA Next Generation Space Telescope, or NGST [James Webb Space Telescope], a near-infrared telescope that will succeed the Hubble Space Telescope. Planet hunting will be a secondary part of its job. Like Hubble, NGST will be a general-purpose telescope with an emphasis on cosmology. But it will investigate stars with dusty disks – the early stage of planet formation – and may also be able to study Jupiter-size planets.

Marcy concluded: "We are witnessing the birth of a new observational science: the discovery and characterization of extrasolar planetary systemsWhat fraction of the twinkling stars you see out there are going to have Earths? We’re going to have the answer in 10 years or less.


Related Web Pages

Star chart
Kepler mission
Space Interferometry Mission
Extrasolar Planets Encyclopedia
Info on the PLANET collaboration
Extrasolar Encyclopedia site in France
Jupiter Mass Planets: Scientific American
Frequent Wet Earths?
Habitability: Betting on 37 Gem
Discovering New Worlds
Star Bright: Part I