The Extrasolar Planet at HD-89744

Recent years have witnessed an explosion of discoveries of planets outside our solar system. To date, scientists have found approximately eighty planets orbiting other stars. This is accomplished by analyzing emissions from other stars for Doppler Shift. If Doppler Shift indicates that the center of mass of solar system is not at the center of the star, scientists see this as an indication that at least one extrasolar planet exists in the system. One of the extrasolar planets discovered orbits the star HD-89744, which is about 40 parsecs, or 130 light years, from the earth. The luminosity of the star is 5.2x10^23 w, which causes it to have an apparent magnitude of 5.7 from the earth. This magnitude value indicates a star that can be seen normally with binoculars or a telescope, but only with the naked eye in optimal conditions.

The Star HD-89744 is a F7 main sequence star. The star is slightly more luminous than our sun, and has a surface temperature of 7200 degrees Kelvin. The star has a mass of 2.7x10^30 kg, which is 1.40 times the mass of the sun. The planet HD-89744b orbits its star at a distance of 1.32x10^11 meters at the semi major axis. This means it orbits the star at a distance that is 88 percent of the Earth’s orbit of the sun. The orbital period of the planet is 256 days, or seventy percent of one earth year. The planets discovery was announced on March 3, 2000, by a group of astronomers led by Dr. Noyles. The planet has a mass of 1.36x10^28 kg, or 2280 times the mass of the earth, or 7.17 times the mass of Jupiter. The eccentricity of the planet is .70.

In order to determine if life may exist on the extrasolar planet HD-89744b, one should know the temperature of the planet’s surface. For the planet’s temperature to be in equilibrium, or remain constant, the energy absorbed by the planet from the sun must be euqal to the energy emitted by the planet into space. The amount of light that falls on the planet from the star is given by the amount of energy output form the star, or luminosity, multiplied by the fraction of the light emitted that intersects the planet, given by the area of the circular cross-section of the planet divided by the surface area of the sphere, centered on the star, that contains the planet. This quantity, multiplied by one minus the albedo of the planet, or the fraction of energy that is reflected off the surface of the planet, results in the amount of energy that the planet receives from the star. The amount of energy emitted from the star has been shown to equal the surface area of the planet, multiplied by the flux constant, multiplied by the fourth power of the temperature of this planet. In setting the energy absorbed equal to the energy radiated from the planet and solving for the temperature, the radius of the planet is cancelled out from the equation. This shows that the temperature of a planet does not depend directly on the size or mass of the planet. We found this equation to be true:

L×(1-A) ^¼ _{= Tp}

16pσd²

This equality allows us to calculate the temperature of HD-89744b, with the exception that we do not know the albedo (A) value for the planet’s surface. The albedo values found for planets in our solar system range from 0.11 for the dirt and rock surface of mercry to 0.65 for the complete cloud cover of Venus. Using various values of albedo within this range, we can determine a range of possible surface temperatures of HD-89744b. There is also a wide gradient of temperature in the course of one orbit, because of the planets high eccentricity of .70. This means that the distance between the star and the planet varies dramatically, and the temperature calculations are based on the distance at the semi major axis, and so can be expected to the coldest because it is at the farthest point of its orbit. However, the variation in distance would mean that the planet could get significantly hotter than these values.

Albedo	Surface Temperature
.11	311°K (38°C)
.2	303°K (30°C)
.3	293°K (20°C)
.4	282°K (9°C)
.5	269°K (-4°C)
.6	255°K (-18°C)
.65	246°K (-27°C)

Knowing the temperature of the planet allows several more things to be determined. Life on earth can survive at temperatures as cold as -13°C, but creation of life becomes more likely as temperatures rise from this point. Therefore, finding life on HD-89744b is dependant on the planets albedo being low, with the .2 to .3 range, which produces temperatures within the range that life flourishes on earth. The temperature could possibly be driven higher than the albedo predicts by the presence of a greenhouse effect trapping heat in the atmosphere of the planet.

The planet’s mass and its postion near the star means that the most probable explaination of its formation is that it migrated inward. Planets that form so close to their star, even closer than the earth, would not have enough material available to form such a great mass because the star would draw most of the material into itself.

Although the temperature ranges for HD-89744b suggest that it may be inhabitable, the high eccentricity of the planetary orbit must be taken into account. The high eccentricity of .70 means that the planet forms a very elongated, “cigar-shaped” orbit around the star, becoming close to the star and traveling faster at one point in its orbit and slowing down as it moves farther away again. Such a violent and rapid periodic fluctuation in orbital radius would cause an equally violent temperature change every orbital period. There are possibly other hazards included with this variation, as bombardment would most likely be more frequent and catastrophic when the planet is farther from its star. As far as can be told without actually seeing the planet HD-89744b, there is a very low probability of life being able to develop on the surface.