A planet orbits a faraway star. Although it’s light it tugs on its parent sun, so that sometimes the star is pulled slightly towards us, and sometimes pushed away. How can we tell? That tiny orbital-wiggle creates a slowly-oscillating Doppler shift in the frequency of the starlight we see.
It sounds like a tiny effect and it is, but HARPS (the High Accuracy Radial Velocity Planet Searcher) can measure stellar radial velocities to an accuracy of 1 meter per second – and that’s enough for some new discoveries. HARPS has just found 16 super-Earths, exoplanets with a mass not much higher than our own. The most promising of the newcomers is the planet HD 85512 b, around 3.6 times the mass of the Earth. Its surface gravity is estimated to be around 1.4 g and its climate similar to the south of France, although some observers suggest it’s a trifle muggy. It’s the best exoplanet yet for the prospects of habitability and at a mere 36 light years away, it’s practically in our backyard.
If we do find evidence for life, the next step will be to take a close-up look and that means a big telescope – in fact a very big telescope! The resolving ability of any telescope is limited by diffraction, which blurs the fine detail: the bigger the diameter of the telescope, the more detail you can see. Suppose we wanted to image HD 85512 b at a spatial resolution of one kilometer where we could just make out their cities. A suitable Earth-orbiting space telescope would have to be more than 5,000 kilometers (3,000 miles) across, wider than the Atlantic; it would take months to build up an image, photon by photon.
A less fanciful option would be to use the Sun’s enormous gravitational field as a lens – as proposed by the FOCAL mission. A telescope at 600 Astronomical Units (fifteen times Pluto’s distance out = three and a half light days) would see HD 85512 b focused by the sun as if it were at the distance of the Sun itself. From the telescope’s point of view, it would be like looking at Mercury. To resolve features at a granularity of one kilometer from that distance out you would need a telescope diameter of around 50 kilometers. It’s certainly quite a bit bigger than the Hubble and the proposed James Webb space telescope! … But a technology which could get a functioning telescope out to 600 AU would certainly be able to build one.
Perhaps we should leave it to the next Administration for funding though!
NASA finds Star Wars’ Tatooine in real-life … kinda
NASA’s Kepler mission has discovered a world with a double sunset, similar to Star Wars’ Tatooine. The new planet, Kepler-16b, orbits two stars at a distance of 200 light-years from Earth.
The NASA press release stated that “Kepler-16b is an inhospitable, cold world about the size of Saturn and thought to be made up of about half rock and half gas. The parent stars are smaller than our sun. One is 69 percent the mass of the sun and the other only 20 percent. Kepler-16b orbits around both stars every 229 days, similar to Venus’ 225-day orbit, but lies outside the system’s habitable zone, where liquid water could exist on the surface, because the stars are cooler than our sun.”
The new planet is lucky not to be ejected from the system: planetary orbits around binary stars are notoriously unstable. Astronomer Eric Mamajek of Pennsylvania State University comments: “The main problem with forming planets in multiple star systems is dynamic ejection… stars can simply toss planetesimals out of the system all together (or even accrete them). An example of this is the Kirkwood gaps in the asteroid belt where the Jupiter-Sun system doesn’t allow asteroids to exist in certain orbits, and conversely “shepherds” asteroids into certain other orbits. A companion star would have a similar effect, except there would be a lot less “shepherding” orbits. … It is difficult for planets to exist in a system with two very massive bodies in an eccentric orbit. They can only exist very close to each star, or very far from both stars.”
In fact Kepler-16b is unusual in that it falls inside the radius that was thought to be the inner limit for planet formation in a binary star system. According to Sara Seager, a planetary expert at the Massachusetts Institute of Technology, it was thought that for a planet to have a stable orbit around such a system, it would need to be at least seven times as far from the stars as the stars are from each other: Kepler-16b’s orbit is only about half that distance. [Wikipedia].