Lecture 16 - Exoplanets (techniques for finding extra solar planets)

what are the two ways to detect exoplents

• direct

• indirect

wha is direct detection of exoplanets

images/spectra of exoplanet(s) themselves

what is indirect detection of exoplents

measurement of star may reveal the effects of orbiting planet(s)

when does direct detection work the best?

• Star system is relatively near us,

• Planet is large (considerably larger than Jupiter),

• Large distance from host star,

• Planet is hot, and hence bright in IR (distant planets orbiting their host star reflect very little starlight, so thermal emission (IR) is detected instead)

what are indirect methods

• astrometry

• transit

• doppler shift

• pulsar timing

• microlensing

what is astrometry

• (oldest method) motion of stars across the sky

• All stars exhibit Proper Motion* due to their individual motion around the galaxy.

• Most PM are small (5-10 km/s) compared to the general galactic rotation (~250 km/s)

• However, nearby stars change their position by a few arcsec**/yr
  

what is transit

planet crossing host star

what is doppler shift

radial velocity of ‘wobbling’ stars

what is pulsar timing

cosmic clocks reveal planets

what is microlensing

general relativity in action

what is proper motion

apparent angular motion of a star across the sky measured with respect to more distant stars. **arcsec = unit of angular measurement; 1/3600 of a degree (c.f. moon has angular size of 30 arcsec.

what does astrometry method rely on

the fact that if there were planets going around a star, then the star's Proper Motion across the sky would be modified from a straight line:

what is the proper motion of a star affected by

gravitational pull of planet: star and planet orbit around their common centre of mass, causing a 'wobble' in the star's proper motion

what is barnards star?

• the 2nd nearest star to us (6 ly)

• For many years ('63-'73) a substantial number of astronomers became convinced that Barnard's Star was showing a wobble in its PM, due to the presence of planets.

• In particular, Prof. Peter v/d Kamp spent 30 years studying it, and produced several papers in which he reported on the “discovery” of 2 Jovian planets. • Regrettably, people no longer believe his result:

• New measurements in 1973 undermined the claim of planet(s) around Barnard's star, and they believed that the wobble was caused by a shift in the telescope lens rather than by PM.

• However, in 2018, a super-Earth like planet was discovered by the radial velocity (RV) technique, but this claim was refuted again in 2021.

• In Aug 2024, by using RV technique a planet of ~0.3 ME and a Period of 3.15 days was confirmed, and 3 candidates proposed (confirmed in March 2025)

what is planetary transits?

A fairly direct (still indirect) technique that relies on the correct orientation of the star system. When a planet transits across its host star we can see a dip in the stars brightness.

what does how often the transit occurs give

the orbital period

what does the duration, depth and shape of the trasnit show

other parameters like the planet/star size ratio

what are the major problems with transits

• The signal drop can be very small.

• We don’t know when it will happen?

• Most star/exoplanet systems won’t be suitable, due to their orientation (inclination of the system). •

• If you discover a Jupiter-like transit (with large orbital period), you will have to wait several years till the next one.

• This method has a high rate of false detections, and a star with a single transit detection requires additional confirmation from other methods.

example of transits ino our solar system

IF we observed our Solar System from 100 ly away... • Then Jupiter would cause the light from our Sun to drop by ~1%, every 12 years if it passed in front of it. • The transit of the Earth however, would only cause a tiny (1/12000) drop in Sun's brightness, but yearly!

how can the transit method be used?

• Can be used (in some cases) to confirm existence of possible candidates, detected by other methods.

• Also makes it possible to study the atmosphere of the planet:

how does transits make it possible to study the atmsophere of the planet

• When the planet transits, starlight passes through the planets’ upper atmosphere.

• Studying the high-resolution spectrum carefully, one can detect elements present in the planets’ atmosphere.

what are doppler shifts

Indirect technique to search for a stars’ orbital movement around its centre of mass by studying its spectrum. Small shifts in the wavelength of spectral lines (blue & red) are caused by velocity changes imparted to a star by having planets going around it

what are benefits of dopler method

• very precuse at deterining the orbit parameters of a planet

• a lot of planets have been discovered using this method

what are the limits of the doppler method

• Best suited for identifying massive planets orbiting relatively close to their host star:

  • Gravity weakens with distance: the closer the planet, the harder the pull and the faster the star moves.

  • Shorter orbital period takes less time to observe the periodic Doppler shifts.

• Current velocity detection limit is ~0.4m/s  So Earth-like planets can just about be detected  However, Jupiter causes 30 m/s motion on our Sun  Even more, if much closer to host star

what is pulsar timing

• host star is a neutron star that spins 161x/s (period = 6 ms).

when was the first planets discovered by pulasr timing

1992

why is pulsar timing good

extremely powerful and precise twchnique

why is pulsar timing bad

it can only find planets around dead stars which beam out hard radiation

when is gravitational microlensing

When a planet happens to pass in front of a star along our line of sight, the planet's gravity will behave like a lens. It focuses the light rays and causes a temporary sharp increase in brightness and change of the apparent position of the star.

Light from a distant object will bent due to the distortion
of space...
In some cases, the intervening mass will act as a lens:
focussing and magnifying the light rays

if the foreground object has a planet, it is possible to measure the contribution of the gravitational field of this planet to the total lensing effect.

what does genral relativity predict

predicts that if a light beam, from a distant object, passes close to a foreground mass, then its gravity will bent the beam from its trajectory due to the distortion of space

what is OGLE

optical gravitational lense experiment inchile

• a polish astronomical project concerned with discovering dark matter

why is OGLE good

• With such a low probability to observe gravitational microlensing it is necessary to observe many stars!

• OGLE observes several million stars each night (main targets: Galactic Bulge or Magellanic clouds) → getting several lensing events/yr.

• Several planets have been discovered by the OGLE project: via transit method and by gravitational microlensing, including gas giants, super-Earths, and even terrestrial-mass planets.

  
  

what is prolem with OGLE

• Can never predict when or where this mircolensing event will happen.

• Can never repeat the experiment to check if it was correct!

what is interferometry

• By combining the signal from two/more separate
  telescopes, the resolving power is greatly increased:

• The planet could be observed separately from the star.

what limits interferometry

atmospheric turbulence