A Hot Earth-Neptune Hybrid

Artist's concept
A red dwarf star shines in the background of a big steamworld in this artist's concept.
Copyright Lynette Cook

With 236 known subjects, extrasolar planetology has matured to the point that most new planet discoveries meet with indifference. But when astronomers deduce unusually detailed information about a new planet, and especially if it belongs to a whole new class, a data point on a graph turns into a real world that stirs the imagination.

Such was the case early this week for the first “hot Neptune” found crossing the face of its host star. The planet, known as Gliese 436b, has about the same mass and size as Neptune. Yet it orbits its star a thousand times closer than Neptune orbits our Sun.

“This is the lowest-mass planet known to transit its star by a factor of four,” Jason Wright (University of California, Berkeley) told a press briefing Monday at the American Astronomical Society’s semiannual meeting in Honolulu, Hawaii. Wright is a member of the California and Carnegie Planet Search team, founded by Geoff Marcy and Paul Butler. The team discovered the planet in 2004 by noticing its gravitational tugs on its host star, the red dwarf Gliese 436 in Leo.

Last week Michael Gillon (Liège University, Belgium) and nine colleagues announced that Gliese 436b transits its star. This tells astronomers that the planet’s minimum mass as found by the gravitational-tug method is virtually the same as its actual mass. And the amount that its silhouette dims the star reveals the planet's diameter. Gliese 436b has 23 times the mass of Earth, slightly more than Neptune (17 Earth masses), and its diameter is also about the same as Neptune’s (4 Earths). The planet’s average density is thus about twice that of liquid water.

Marcy likens the body to a “hybrid super-Earth / Neptune.” With a size and density comparable to Neptune’s, Gliese 436b likely has a similar interior structure. Astronomers conjecture that it has a rock-and-iron core surrounded by a thick mantle of water that is compressed by high pressure into solid form despite the world's internal heat. The water mantle may contain more than half of the planet’s mass. And like Uranus and Neptune, it almost certainly has an outer atmosphere mostly of hydrogen and helium.

But the planet orbits Gliese 436 (a low-mass, low-luminosity star 30 light-years from Earth) at only 3% of Earth’s distance from the Sun. It's so close to the star that it completes an orbit every 2.644 days.

Surprisingly, the orbit is mildly elongated (eccentricity 0.16). This is unusual for a planet so close to its star; normally tidal interactions with the star would work such an object into a circular orbit. But Wright noted that observations of the star’s wobble strongly suggest the presence of a second planet. Gravitational perturbations from this other object probably pull Gliese 436b’s orbit into an ellipse. If this other object is confirmed, Gliese 436b would be the first transiting exoplanet known to be in a multiple-planet system.

Sadly, the tilt of the known planet's orbit is high enough that any worlds orbiting farther out are extremely unlikely to transit the star.