Weeks ago, an announcement about the discovery on an exoplanet where it appeared possible for an atmosphere with precipitation to exist led it to be described as an “earthlike planet,” and suggestions were made about its potential habitability.

Very little was said about its structure being far more like Jupiter mostly gas with a tiny rocky core very far below the cloud tops. That’s an important component of the composition in a description mentioning ‘habitability’ and ‘earthlike’.

• Mass determines if the object produces a gravity field capable of holding an atmosphere for some length of time. Venus and Earth are massive enough, Mars was only able to hold a thick atmosphere for a comparatively short period early in its history.
• Position relative to their host star: the ‘Goldilocks (habitable) Zone’ ideally looking for a distance where water can exist as a liquid.
• Density of exoplanet candidate ‘Hot Jupiters’ have been identified as Jupiter-sized objects (big size, low density) with short orbital periods orbit close to their host star.
• What type of star is hosting the exoplanet(s)? Cool, mid-temp or hot(Lo mass vs hi mass)?

This has implications for the composition of the planet and the dynamics of planetary formation in that star system. We are as yet unable to gather enough data to clearly identify something as TRULY earthlike. We can use only inference to determine mass, density, size and transmission spectroscopy to get any kind of handle on atmospheric composition.

Turning to night sky highlights this week: Jupiter and Saturn still hold court in the South/Southwest evening sky. The moon is at its new phase today so look for a thin crescent emerging from the west just after sunset tomorrow, Sunday, and Monday evenings.