Fifteen years ago, astronomers knew of only eight planets in the universe (nine if you still count Pluto). Now nearly 400 planets outside our solar system populate an ever-increasing planetary catalog. Recent advances have revealed the first definitive detection of a rocky planet and the first direct observation of a planet-like object orbiting near a sun-like star. While the catalog of extra-solar planets grows, so does the list of requirements the planetary system must meet in order to support life. As astronomers continue to find new planets using existing and novel technologies, the body of evidence showing the uniqueness of our solar system to support life also grows larger.
SOLAR SYSTEM STILL LOOKS RARE
This past year witnessed two firsts in extrasolar planet research. The European HARPS collaboration discovered the first definitively rocky planet around a star named CoRoT-7.1 A different team directly imaged the first planet-like object orbiting in close proximity to a Sun-like star.2 These two discoveries reflect astounding achievements over the last 15 years in astronomers’ abilities to detect planets. Yet the details of the detections continue to reveal decisive differences between the solar system and all the discovered extrasolar planets.
The planet discovered around the star CoRoT-7 stands as an example. Using the radial velocity technique (link to DETECTION TECHNIQUES), the HARPS collaboration found a planet, named CoRoT-7b, with a mass only five times the mass of Earth (they also discovered another planet with a mass eight times Earth’s). Transits across the star by the planet revealed that its radius was only 80 percent larger than Earth’s. Thus, CoRoT-7b has an average density of 5.6 g/cm3 compared to Earth’s density of 5.5 g/cm3. These observations conclusively demonstrate the rocky nature of this planet. But, outside of exhibiting a circular orbit, the similarities to Earth end there.
For example, CoRoT-7b makes a revolution every 20 hours (the other planet candidate in the system orbits the star every 3.7 days) compared to Earth’s 365 days around the Sun. The star-planet distance derived from this orbital period means that CoRoT-7b sits 23 times closer to its star than Mercury does to the Sun. Consequently, the temperature on the star-facing side of the planet exceeds 2000oF—enough to liquefy the rocky surface. In contrast, the opposite side of the planet sees temperatures around -200oF. Neither situation permits life in any form.
The directly detected planet also differs markedly from objects found in the solar system. All previous direct detections involved objects that were either separated by distances ten times larger, orbited extremely large or small stars, or exhibited star-like temperatures. The notable aspect of this direct observation is that the planet has an orbital size and eccentricity similar to that of Pluto. Additionally, its temperature is a few hundred degrees (planet-like) instead of a few thousand degrees (star-like). Once again, the similarities end there.
Because of uncertainties in the host star’s age, the mass of the imaged planet ranges from 10 to 40 times the mass of Jupiter. Even at the low end of the mass range, the eccentricity of this object and its mass would disrupt the orbits of any smaller planets in the system. In fact, radial velocity searches performed on this star have not detected any planets closer to the star than the directly detected planet. If the object had a mass at the high end of the range, it would be classified as a brown dwarf or “failed star.” Brown dwarfs are more like stars than planets except they were not large enough earlier in their history to sustain nuclear fusion. Either way, when all the details are considered, this planet-like object does not resemble any solar system object.
Breathtaking advances in planet-finding technology over the past fifteen years have revealed hundreds of planets outside the solar system. The scientific knowledge base expands with each discovery, and so does the opportunity to affirm a conclusion about Earth and the solar system. These planets exhibit a diverse set of characteristics that continue to buttress the notion that the solar system’s capacity to support life may be unique because it is the product of a Divine Designer.
Astronomers use a number of techniques to detect planets around other stars, and each method has advantages and limitations.
Radial Velocity: The most prolific planet-finding technique. It looks for the gravitational tugs on the star as a planet orbits. This method permits astronomers to determine the planet’s mass and orbital characteristics but little else.
Transits: Astronomers look for dips in the amount of detected starlight resulting from a planet passing in front of the star. Transits allow astronomers to determine the orbital characteristics, planet size, and mass. Occasionally, light from the planet itself can be determined from transits. While more information comes from transits, they occur less frequently. The Kepler mission (https://kepler.nasa.gov/) uses this method.
Gravitational Lensing: Occasionally, a star and an associated planet will pass in front of a background star. For specific alignments, the star and planet gravitationally lens (enhance the view of) the background star, causing a brief but dramatic increase in detected light. While gravitational lensing searches have detected only a dozen or so planets thus far, it is the only technique capable of finding Earth-mass planets around stars with masses similar to the Sun.
Direct Detection: This method seeks to directly detect the light coming from an extra-solar planet. Because the light from the star dwarfs the planetary light, this technique currently detects only Jupiter-class planets orbiting relatively far (more than 10 times the Earth-Sun distance) from their host stars. However, the light from the planet carries much information about the planet size, temperature, orbit, and atmosphere. The planned Terrestrial Planet Finder mission seeks to directly image an Earth-like planet orbiting in the water habitable zone around a Sun-like star.