What may turn out to be a remarkable discovery was made recently concerning the possible atmosphere of an extrasolar object orbiting a star similar to our Sun, but located 150 light-years from our Solar System. The object might actually be a planet, in which case the discovery of gaseous sodium in its extremely hot atmosphere would be an important finding in opening avenues to discover other gasses on other extrasolar planets that could indicate the possibility of life elsewhere in space. Although previously known, the distant star was not considered important enough to be given any name other than a catalogue number, HD-209458.

The discovery was made using the versatile Hubble Telescope that orbits Earth far above our atmosphere which both limits and obscures the operations of all ground-based telescopes. Astronomers used a spectrograph on Hubble to find gaseous sodium because it was the easiest atmospheric constituent to detect, an important consideration in pursuing an astronomical breakthrough.

Because the Jupiter-sized object circles HD-209458 once every three-and-a-half days, it must be too close to its sun-like star to be considered a candidate for having life on it. But fortunately, the plane of its orbit was just right for the object to pass exactly between its star and Earth. This enabled scientists to view HD-209458's partially diminished starlight when the light passed through the object's peripheral atmosphere. The spectrograph was able to distinguish sodium from among other as yet unknown atmospheric constituents. But is the object definitely a planet? Scientists cannot tell with absolute certainty. As lawyers would say in court cases where circumstantial evidence is the best information available, the data here indicate with a reasonable degree of certainty that the object is a planet.

The discoverers claim that improved technologies will allow other atmospheric gasses to be discerned on other extrasolar planets. And if carbon dioxide, water vapor, methane and ozone are detected, they say these would indicate the strong probability that life exists there.

There is a slight possibility, however, that the object orbiting HD-209458 could be a small, low-mass, dim star which is part of a binary stellar system with a larger, massive, brighter star. Because they both contain mass, the small and large masses actually orbit each other about a common center known as the barycentric point which is closer to the larger mass (much as a heavy and a light dancer holding hands while whirling about each other).

When the orbital plane enables it to occur (whether the smaller object is a planet or a star), there is a marked diminution of light visible from the large star each time the small object passes directly between the brighter star and Earth (much like a solar eclipse, when the Moon passes directly between the Sun and viewers on Earth, causing a temporary darkening).

Although the Moon's relative size and distance enables it to cast a large shadow that may sometimes obliterate the Sun for some viewers, two familiar Solar System planets, Mercury and Venus, periodically pass directly between the Sun and small areas on Earth. Both planets are too small and orbit too close to the sun to then create large encompassing eclipses. Instead, they both appear as tiny dots racing separately across the large visible face of the Sun. Such an event is known as a transit, which is less common for Venus than for Mercury.

Whether a partial eclipse or a transit, we may have to wait decades or centuries before life can be confirmed on extrasolar planets elsewhere in the Universe.