L1 - Extrasolar Planets I Flashcards
Define what an Extrasolar Planet (Exoplanet) is.
An Extrasolar Planet (Exoplanet) is a planet which orbits a star – or remnant of a star – beyond our Solar System. WGESP, defined as planets, those bodies: With masses below the limiting mass for thermonuclear fusion of D, i.e. ~13 MJ, (where MJ is mass of Jupiter) for objects of solar metallicity.
What are the five different techniques employed by astronomers to detect astrophysical bodies?
Reflected Radiation Emitted Radiation Absorbed or Occulted Radiation Refracted Radiation Effects on the motion of nearby matter
Explain the Reflected Radiation technique.
Measuring the electromagnetic radiation reflected by the exoplanet (similar to emitted radiation).
Explain the Emitted Radiation technique.
Measuring the electromagnetic radiation emitted by the exoplanet (similar to reflected radiation).
Explain the Absorbed or Occulted Radiation technique.
As an exoplanet passes in between its host star and Earth it dims the host star as seen from Earth.
Explain the Refracted Radiation
technique.
Light can also be refracted close to massive bodies (stars, planets). Gravitational micro-lensing causes a background star to brighten and fade over weeks when a foreground star (lens) passes in front of it. The presence of an extrasolar planet may be detected as a multi-day peak on top of the star’s micro-lensing time profile.
Explain the Effects on the motion of nearby matter technique.
The presence of a planet will influence the orbit of a star. Doppler spectroscopy (Doppler Detection Method) or Radial Velocity Method, which measures a star’s variable velocity along the line of sight. The Astrometric Detection (“wobble”) Method (Astrometry) can be employed in the search for Extrasolar Planets.
Advantages and Disadvantages of Reflected Radiation technique?
The problem using reflected light to identify Extrasolar Planets is the enormous brightness ratio between the star and planet. The planet and star are very different in brightness and magnitude and so they cannot be easily resolved. Note that the ratio of the Sun’s brightness to that of Jupiter’s brightness as measured from another star would be roughly a billion to one. Better for more massive planets further from the host star.
Advantages and Disadvantages of the Emitted Radiation technique?
Infrared (IR) radiation emitted by the planet is more promising as brightness ratio is much smaller. In the case of the Jupiter-like planet around a nearby star, the IR brightness ratio can be reduced to one million to one, or better. IR observations of Extrasolar Planets atmospheres is difficult, but would allow detection of biosignature gases, e.g. H2O, O3, CH4, etc. Better for more massive planets further from the host star.
Advantages and Disadvantages of the Absorbed or Occulted Radiation technique?
Only possible when planet’s orbit takes it between the star and the Earth. A light-curve shows astronomers how brightness varies with time and the size of the dip allows an estimate of the radius of the planet. A knowledge of the mass permits an estimate of density (terrestrial, Gas Giant, etc.). Better for more massive planets near the host star.
Advantages and Disadvantages of the Refracted Radiation technique?
Micro-lensing the only technique known with potential of detecting extrasolar planets at centre of Milky Way, but micro-lensing events are not reproducible. They depend on the alignment of:
• Observer (on/near Earth)
• Source (star)
• Lens
Micro-lensing techniques also lack accuracy when estimating the distance of distant stars and Extra solar Planets.
One advantage is that the intensity of the planetary deviation does not depend on the planet mass as strongly as effects in other techniques do. This makes microlensing well suited to finding low-mass planets. It also allows detecting planets further away from the host star than most of the other methods. One disadvantage is that follow-up of the lens system is very difficult after the event has ended, because it takes a long time for the lens and the source to be sufficiently separated to resolve them separately.
Advantages and Disadvantages of the Effects on the motion of nearby matter technique?
Can tell us about the nature of the orbit of the ‘invisible’ planet. Better for more massive planets near the host star. The major problem with the Doppler method is that it can only measure movement along the line-of-sight, and so depends on a measurement (or estimate) of the inclination of the planet’s orbit to determine the planet’s mass.
