Part of the Exoplanets exhibition.

Discovering a Dwarf

Brown Dwarf Gliese 229B
Brown dwarf Gliese 229B (tiny dot), discovered with the coronagraph in the exhibit

Below the large star in this image is the brown dwarf Gliese 229B, a dim object too small to be a star yet too large to be a planet. Using the coronagraph in the exhibit, AMNH astronomer Rebecca Oppenheimer, together with researchers from California Institute of Technology and Johns Hopkins University, discovered this brown dwarf, the first one ever seen. The coronagraph revealed the brown dwarf by blocking out most of the star's light.

Dimming Starlight
Telescopes can easily zoom in on a star. But to see a dim object next to a star, such as a planet, astronomers need to block out the star's light. A coronagraph attached to a telescope can eliminate nearly all the starlight, revealing dim objects nearby. With this coronagraph, researchers made the first discovery of a starlike object called a brown dwarf. Today coronagraphs are helping astronomers search for new planets.

Lyot Dwarf image
Lyot Project coronagraphic image of possible brown dwarf (dot at left)

Closer to the Star
Better coronagraphs will soon allow researchers to look for planets around other stars. The Lyot Project coronagraph, built at this Museum, has already detected a starlike object—possibly a brown dwarf—about as far from the star as Pluto is from the Sun.

coronagraph diagram

How It Works:
Inside the coronagraph, a series of mirrors and lenses position and focus the light. Carefully placed disks and rings block out 98.5% of the star's light, enabling astronomers to see dim objects near the star.

1. Light enters from telescope at top
2. Mirror reflects light upwards (visible at back of instrument)
3. Mirror reflects light downwards 4. Reflective central disk (center of lens, below left) blocks 95% of starlight
5. Outer ring and central disk (object above right) block light that bends around central disk. 
6. Image reflected to right
7. Image reflected upwards
8. Camera receives final image (visible at right side of instrument)

Hidden Planets 
If you looked at our Sun from a telescope 30 light years away, its light would be so bright (left) it would obscure all the planets. Using a telescope with a coronagraph, Jupiter and Saturn would become visible (right).

AEOS Solar System
© R. Makidon, A. Sivaramakrishnan, B.R. Oppenheimer/AMNH
telescope greyscale
Telescope at the Palomar Observatory

A coronagraph on display in the exhibition was attached to the 60-inch (1.5-meter) telescope at the Palomar Observatory in southern California. With it, researchers made the first discovery of a brown dwarf, as well as several "debris disks" where planets take shape.


The Future of Finding Planets
Armed with the latest interferometers, coronagraphs and other technologies, astronomers will soon capture the first images of faraway planets.

coronagraphic space telescope
Artist's rendering of a coronagraphic space telescope. This is the concept for an ambitious new space observatory designed to see Earth-like planets around nearby stars.
© Navigator Program, NASA

From Earth
Today's advanced coronagraphs are close to being able to image hot young planets orbiting nearby stars. The Lyot Project Coronagraph is powerful enough to see objects a million times fainter than a star and extremely close to the star.

From Space
To search for Earthlike planets, astronomers hope to launch an interferometer and a coronagraph into orbit where the images won't be distorted by Earth's atmosphere. These instruments could study light from an Earthlike planet to look for oxygen, water and other signs of life.