Is That Tiny Dot a Planet?

Click to enlarge NASA image of Hubble Space Telescope

Traditionally, potential planets around distant stars have been detected by a tell-tale wobble in a star's spectrum caused by the tug of the mass of an orbiting planet. Nobody wanted to look directly at a planet because it was thought that stars are so bright, compared with planets, the weak light reflected from a planet would be washed out.

While that might be the case for planets very close to a star, some planets might be far enough away from their stars to be visible. To check out that new idea, astronomers used the Hubble Space Telescope's coronagraph -- an instrument which blocks the light from a star in the center of a telescope's field of view, permitting dimmer objects further away from the center to be seen.

When the coronagraph was used to blot out the star TWA 6B – 200 light years away – the astronomers spotted a tiny dot that looked like a planet at about 150 astronomical units (AU) from the star.

A lightyear is about 6 trillion miles, the distance light travels in a year. An AU is the distance from Earth to the Sun, about 93 million miles.

There is a possibility that the planetary-candidate dot might be nothing more than a background star that happened to lie along the line of sight to TWA 6B. However, the dot object is very red. Stars so red are rare. Astronomers suggest that, if it is a newly formed planet, it might still be releasing heat as it contracts under the weight of its own gravity. That could give it a red-hot glow.


Dust Rings at Two Other Stars
The same tream of astronomers that found the dot near TWA 6B also found rings of dust around two other stars.

Dust rings around a star suggest that unseen planets lurk there. Astronomers say it takes a planet's gravitational pull to organize dust into a ring, in much the same way the rings around planets in our Solar System are produced by so-called shepherd moons.

HR 4796A. Hubble's picture of the young star HR 4796A showed a single thin ring of dust shining as it reflected the star's light.

One way for dust to concentrate into such a ring, astronomers suggest, is for a pair of small planets to be travelling in orbits inside and outside of the dust ring. From their positions, they could shepherd the dust between them. By comparison, two of Saturn's moons, Prometheus and Pandora, were discovered by searching for the shepherds of one of the big ringed planets thin rings.

The rings around HR 4796A suggest that at least one planet, maybe two, are in the vicinity.

HD 141569. Using Hubble's picture of the star HD 141569, astonomers deduced the existence of a planet from the absence of dust within a thin black ring of an otherwise completely-filled-in disk of dust.

The image of HD 141569 showed a gap reminiscent of the large gaps in Saturn's rings. Either a planet is inside the gap in the HD 141569 disk -- sweeping up the dust as it orbits like a finger on a dusty table – or else a planet is orbiting outside the ring and its gravity is ejecting the dust from the ring.

Whether a dust ring or a ring of no dust, the orbits of the suspected planets can't be calculated exactly. However, the radius of one ring is 70 AU and the other 225 AU. Like the dot near TWA 6B, they are further away from their parent stars than had been thought possible. By comparison, our planet Pluto is less than 40 AU from the Sun.

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