The Definition Of Earth-Like For Extrasolar Planets

Topic(s): space,science
2009-12-16 23:14:01 US/Pacific

It seems like each week now a science team somewhere announces the discovery of a new extrasolar planet. The most recent announcement is GJ 1214b. I think the discovery is great. But what bothers me slightly is the description of GJ 1214b. It's described as Earth-like. Even though it weighs in at a hefty 6.6 times that of Earth, and even though its radius is 2.7 times that of Earth.

As I've been reading the news of extrasolar planets over the years I've noticed various authors using the term Earth-like and sometimes also the term Earth-twin in a -- if I may be non-technical here -- in a willy-nilly fashion.

I have yet to find a formal definition for either Earth-like or Earth-twin. Yet, It seems like with all the recent discoveries and with the Kepler Mission's potential next year to announce the discovery of thousands (literally!) of extrasolar planets, it seems like we really ought to have formal, mathematical definitions.

I earnestly searched for a scientific guide on what qualifies as Earth-like or Earth-twin. The closest I've found is a PDF copy of a book written in 1964 by the RAND Corporation, Habitable Planets For Man. After taking information from that book and performing some research and analysis, I've put together my formal definitions.

First, I've expanded on the two Earth categories. I've defined four categories which I'll formally define shortly:

1. Earth-like
2. Earth-similar
3. Earth-twin
4. Earth-habitable

Second, I've defined five criteria:

A. Mass - a candidate planet must be within 50% of Earth's mass (M_E = 5.97 x 10²⁴ kg).
B. Radius - a candidate planet must be within 50% of Earth's radius (R_E = 6371 km).
C. Mean surface temperature - a candidate planet must be within 25% of Earth's mean surface temperature (T_Emst = 287 K).
D. Breathable atmosphere - a candidate planet must have a breathable atmosphere.
E. Liquid water - a candidate planet must have accessible liquid water.

The formal definitions for the four categories listed above:

1. Earth-like - meets any two of the five criteria.
2. Earth-similar - meets any three or four of the five criteria.
3. Earth-twin - meets all five of the five criteria.
4. Earth-habitable - meets at least criteria C, D, and E but not necessarily A or B.

How does my guide work for the planets we know well? Mars meets criteria B and C. Venus meets criteria A and B. Thus, both Mars and Venus are Earth-like. If at some point liquid water (criterion E) is discovered on Mars, it would then be upgraded to Earth-similar.

GJ 1214b does not meet A or B, and we don't yet know if it meets C, D, or E. Thus it is wildly premature to call it Earth-like, although we cannot yet rule out the possibility.

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