No, it’s *not* the smallest exoplanet found!

Contributed by
Apr 10, 2008

The web has been buzzing over what everyone is calling the smallest extrasolar planet found, weighing in at about 5 times the mass of the Earth.

Problem is, that's not the smallest exoplanet found, not by a long shot. That record is still held by three planets massing 0.02, 3.9, and 4.3 times the Earth's mass, orbiting the pulsar PSR B1257+12.

This newly found planet, orbiting the red dwarf GJ436, is the smallest yet found orbiting a Sun-like star.

I'm not picking nits here. I'm trying to be careful. The news tends to focus on planets found orbiting stars like the Sun (that is, fusing hydrogen in their cores as the Sun does) because those planets are the ones in environments most like ours. Certainly, those ones resonate with us, because one of the goals of this search is to find not only a planet with roughly the same mass as Earth, but one on which the conditions may be ripe for life to form and evolve.

Trust me, I'm all for that! I'm excited, terribly excited, about this search. I was a part of it, briefly, years ago, and it means a lot to me. But I'm also an astronomer, and I think it's important to remember that not only were terrestrial-massed planets found first, they were found a full three years before any other exoplanets were found orbiting Sun-like stars.

Pulsars are dead stars, the collapsed cores of massive stars that exploded. The three planets in the system found in 1990 are probably nothing like Earth; they either existed before their star went supernovae, in which case they got fried but good, or they accreted after the explosion and formed under very weird conditions that make them exotic. Either way, they're damn odd (to quote Kirk), and so they don't work their way into our minds as well as more familiar planets.

But still, their discovery was a huge advance in astronomy! Who could have thought planets could either survive, or form after, something as devastating and titanic as the explosion of an entire star? It rocked astronomy when they were found, and their existence is still something of a mystery. How did they get there? What are they like? What does a planet look like when it's baked by intense X-rays and particle radiation for thousands of years?

I don't mean to detract from either the search for exoplanets around Sun-like stars (again, big supporter of it) or people writing up the news of such (obviously, also a big supporter). I just want to make sure the record is clear. It pays not to gloss over details sometimes, because in many cases that's where the real treasure and real wonder lies. I don't want folks to forget our prejudice toward Earth-like (or solar system-like) planets, when the exotic and bizarre might get overlooked.

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