More Method Notes

[aesmael]

Questions, please. Anything I can clarify will help me to learn and help me to present. These notes are based mostly on the paper Astronomical Searches for Earth-Like Planets And Signs Of Life by Neville Woolf and J. Roger Angel. It dates back to 1998 so much may be out of date by now.

Lensing -
    Off-the-cuff summary (Cuffs off! Yucky formal boy clothes!): Like all things light is affected by gravity. The presence of mass bends its path and if the situation is right a massive object can act as a lens, focusing light from an object in the background in our direction. What we are specifically interested in is when a star in the foreground passes in front of a star in the background and the lensing effect temporarily brightens the background star.And sometimes, just sometimes, if the background star has a planet we will see a secondary brightening from that too.

• I am referred to the papers Peale 1997 & Paczynski 1996 for more information
• Looking in the direction of the galactic centre the rate of microlensing events observed is 1 per day. It is suggested that 1 in 30 will show the presence of a giant planet
• To find an Earth-like planet would require very high time resolution and telescopes dedicated to the search
• Finding large numbers of giant planets would indicate there should be terrestrial planets too, as they are believed to be easier to form. The authors are careful to note, though, that 'should' =/= 'is'

I am going to leave the rest for now because I am tired and need to get up tomorrow. Est. 20 days to finish.

Comments

[lantyssa.livejournal.com]

Which of the techniques would work best in concert to identify planetary systems, or are the nature of each technique better suited to finding different types of planets?

Can you give a quick pro/con summary for each technique?

[aesmael.livejournal.com]

Thanks for the questions! Answering backwardsish, most techniques are biased toward finding larger more massive planets.

The radial velocity method best finds planets which are massive and close in to their stars. One main problem with this are fluctuations in the star's surface which put a fundamental limit on what we can detect. Another is that when a planet is found with this method we usually have no idea how its orbit is oriented relative to our line of sight so we can only express its mass as a relation. If you have been seeing in the news reports of 'we have found a planet of such-and-such mass' this is actually almost always a minimum value and it could actually be much higher. For a few years this allowed people to claim that what was being found were actually brown dwarfs and not planets but the statistics of what was being found very quickly made this look very unlikely.

Astrometry best finds massive planets in more distant orbits but you need to keep up your observing program for a long time (for example, Neptune's period is 164 years). For now it is only a hypothetical technique.

Transits are more egalitarian than most other methods - it can more easily detect smaller-sized planets than any other method but it still has a preference for larger, close-in worlds. The major disadvantage of this method is a planet's orbit needs to be in our line of sight to the star for us to find it with this method so we will only find a small fraction of planets from transits. This can be helpful too, since if we know a planet transits we know roughly the inclination of its orbit and follow-up radial velocity measurements can determine its mass much more precisely.

Gravitational lensing I don't know much about yet which is why I started with it, same with direct detection methods. And in fact I have to run off to work now so I will have to leave it here for now.

Thank you again, this is a great help. :-)