The Faint and the Bright

Click here for larger image. The Sun, a typical G2V dwarf. G stars are characterized by the presence of metallic lines and weak hydrogen.
Credit: Harvard University

Maggie Turnbull, an astronomer with the Carnegie Institution, has spent many years thinking about what kind of stars could harbor Earth-like planets. Her database of potentially habitable star systems could be used as a target list for NASA’s upcoming Terrestrial Planet Finder (TPF) mission.

Turnbull presented a talk, “Remote Sensing of Life and Habitable Worlds: Habstars, Earthshine and TPF,” at a NASA Forum for Astrobiology Research on March 14, 2005.

This edited transcript of the lecture is part three of a four-part series.
(Part 1 * 2 * 3 * 4).

From the TPF engineer’s perspective, we scientists are asking for some very challenging technology, and ultimately we may just have to settle for what the engineers can reasonably build. So the questions are: Of the 500 scientifically interesting stars within 30 parsecs, how many habitable zones will we be able to image? If there’s a planet in those habitable zones, how detectable will that planet be?

To answer those questions, we first have to think about two different kinds of star brightness: intrinsic versus apparent.

Some stars look brighter because they are hotter and more massive than other stars. The big blue O and B-class stars, for instance, are intrinsically brighter, or more luminous, then a G star like our sun. The more intrinsically luminous a star is, the more it will swamp the light from a planet in the habitable zone.

The red colors of Orion.
Credit: A. Vannini, G. Li Causi, A. Ricciardi and A. Garatti

On the other hand, faint, cool stars can still appear bright in the sky because they are nearer to us, being more common in the galaxy than intrinsically bright stars. These nearby stars are apparently bright (in the sky) but intrinsically faint (in reality). Planets orbiting intrinsically faint stars can be easier to image because the reflected light from the planet is not overwhelmed by the star’s light.

Within about 10 years, engineers expect they will be able to design a coronagraph that can see a planet that is one-ten-billionth as bright as the star it orbits. With that limit, to image a planet at the inner edge of the habitable zone, we will have to look at stars that are intrinsically less luminous than two-and-a-half times the luminosity of the sun. For the outer habitable zone, where any planets will be reflecting even less starlight, the stars have to be fainter than about half the luminosity of the sun.

Lucky for us, these intrinsically fainter stars happen to be the most common stars in the universe.

The other factor is that we want our intrinsically faint stars to also be apparently bright. The greater the apparent brightness of a star, the greater the angle will appear to be between the star and its habitable zone, and the better we can resolve the star’s habitable zone.

Blue stars in the Pleiades. These stars produce more UV radiation than red stars.
Credit: DSS and LTImage

The engineers tell us that we will be able to observe habitable zones that, looking from Earth, have an angular size of 40 milliarcseconds or larger. Forty milliarcseconds is about the size of a dime seen from 16 miles away, so the engineers are doing pretty well here — this is the cutting edge of astronomical imaging technology.

In terms of apparent brightness, that 40 milliarcsecond habitable zone translates into a six-and-a-half magnitude star or brighter to resolve the whole habitable zone, or a seven-and-a-half or eighth magnitude star to observe at least the outer part of a habitable zone.

So out of all the stars within 30 parsecs — about 2400 stars in the Hipparcos Catalogue — that leaves us with 105 stars where an Earth-like planet in the inner habitable zone will be detectable. Those stars are G stars, late K stars, and some late F stars: stars that are similar to the sun. For the outer habitable zone, we’re going to be looking at slightly fainter stars: late G stars and early K type stars.

If I cross reference those stars with the list of the 500 habitable stars systems that I would like to observe as a scientist, I come up with 56 Habstars that are at least partially observable with TPF. However, only 10 of those are fully observable, meaning we can image the full habitable zone with TPF and discover a planet. I think those 10 stars are definitely our most interesting targets, and should be in the core TPF target list.

We’re building a stellar database now so that astronomers can go online and play around with making their own target lists for TPF, using the information on the locations of habitable zones, variability, companions, metallicity, and so on.

But we still have many unanswered questions, such as, “How much do the spectral signatures of a planet vary over time?” Also, “How would they vary if you take Earth and put it around a star of a different spectral type?” There’s a lot of basic data that just doesn’t exist in the literature right now, so there’s more that we need to do.

Related Web Pages

Speeding Up in the Zone
Star Light, Star Bright… Any Oxygen Tonight?
How To Find An Extrasolar Planet
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Kepler Mission
Darwin Mission
Space Interferometry Mission
Voyager: Beyond the Great Beyond
Fire and Ice
Beyond Pluto: Ice Planet