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  • Writer's pictureMatt Russell

#235 - Alok Jha - Exoplanets

Updated: May 3, 2021

Linn and Matt talk about exoplanet detection, indirect and direct, Matt chats to Alok Jha about life on Exoplanets and much more, Then Matt and Lin talk about a recent paper that suggests we might detect Dark Matter using Exoplanets.

It’s human nature to stretch, to go, to see, to understand. Exploration is not a choice, really; it’s an imperative.
Michael Collins
  • Born October 31, 1930 in Rome, Italy

  • The same year as Scotch tape was invented and Pluto was discovered.

  • In Summary Collins is a brave, bangout pilot, professional beyond belief, modest sensitive family guy. ... a freakin legend!!!!

  • See Episode 142 as Astronaut of the Week.

  • On April 28, 2021, Collins died from cancer in Naples, Florida, at the age of 90



Alok Jha is a journalist and broadcaster and is the science and technology correspondent at The Economist and author of The Water Book

He has been a science correspondent for ITN and the Guardian, presented science programmes for BBC2 and BBC Radio 4. Alok received a science-writing award from the American Institute of Physics in 2014, was named European Science Writer of the year in 2008. The search for ET hots up - If life exists beyond the solar system, science may find it soon


Exoplanets Direct detection.

We’ve talked in previous episodes about the detection of exoplanets, planets that go round stars other than our own. The two mains ones being

  • Using the Transit Method: the minuscule dimming of the star as a planet passes in front you can infer the radius of that object,

  • Using the Radial velocity method, how much a star is being pulled about by the orbiting planets using Doppler shifts of light (colour change as the star is being pulled towards or away from us) you can infer the mass of that planet.

  • If you know both the mass and the radius you can start to work out the density of a planet, so tantalising information on the physical nature of the planet can be obtained.

There are a few other methods - my 4 favourites

  • Reflection and emission modulations. If a planet has a high albedo as it goes around the star it various phases (like we see venus or the moon) will generate increases and decreases in light level. This may actually yield the most results as it doesn’t matter what orbital plane you are on!!!! The first-ever direct detection of the spectrum of visible light reflected from an exoplanet was made in 2015 by an international team of astronomers. The astronomers studied light from 51 Pegasi b – the first exoplanet discovered orbiting a main-sequence star (a Sunlike star), using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the European Southern Observatory's La Silla Observatory in Chile

  • Relativistic beaming - Mass of a planet pulls the star making it denser at the surface, like pinching a baby's cheek and the denser star will be brighter causing variation in brightness.- really hard but the first discovery of a planet using this method (Kepler-76b) was announced in 2013

  • Pulsar timing - Pulsars are like super-accurate clocks in the sky, so anything perturbing their motion is actually pretty visible as it alters the timing of the clock, this in fact is the most accurate method and the first to reveal a result, In 1992, Aleksander Wolszczan and Dale Frail used this method to discover planets around the pulsar PSR 1257+12.[37] the first confirmation of planets outside the Solar System didn’t get the Nobel curiously. I guess because these planets are so useless and ultimately rare

  • Gravitational microlensing - Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned. Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other. More than a thousand such events have been observed over the past ten years - This was the first method capable of detecting planets of Earth-like mass around ordinary main-sequence stars, which cannot be repeated though. The main advantages of the gravitational microlensing method are that it can detect low-mass planets (in principle down to Mars mass with future space projects such as WFIRST

But what about just plain and simple direct detection, actually seeing the thing in the telescope, not just inferring it is there!!!

  • Least distant directly visible Fomalhaut b 25 light years Also first directly imaged planet at optical wavelength.

  • The problem is Planets are just such extremely faint light sources compared to stars, and worse what little light comes from them tends to be lost in the glare from their parent star.

  • Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their thermal emission instead.

  • Obviously It is easier to obtain images when the star system is relatively near tous, and when the planet is considerably larger than Jupiter, widely separated from its parent star, and hot so that it emits intense infrared radiation;

  • Coronagraphs are used to block light from the star, while leaving the planet visible.

  • Direct imaging of an Earth-like exoplanet would require extreme optothermal stability.

  • During the accretion phase of planetary formation, the star-planet contrast maybe even better in H alpha than it is in infrared – an H alpha survey is currently underway

  • Direct imaging can give only loose constraints of the planet's mass, which is derived from the age of the star and the temperature of the planet. But this Mass can vary wildly, as planets can form several million years after the star has formed.

  • The cooler the planet is, the less the planet's mass needs to be.

  • It is occasionally possible to give constraints to the radius of a planet based on the planet's temperature, its apparent brightness, and its distance from Earth.

  • Good news though, The spectra emitted from planets do not have to be separated from the star, which eases determining the chemical composition of planets.

  • Sometimes observations at multiple wavelengths are needed to rule out the planet being a brown dwarf.

  • Direct imaging can be used to accurately measure the planet's orbit around the star. Unlike the majority of other methods, direct imaging works better with planets with face-on orbits rather than edge-on orbits, as a planet in a face-on orbit is observable during the entirety of the planet's orbit, while planets with edge-on orbits are most easily observable during their period of largest apparent separation from the parent star.

The planets detected through direct imaging currently fall into two categories.

  1. planets are found around stars more massive than the Sun which are young enough to have protoplanetary disks.

  2. consists of possible sub-brown dwarfs found around very dim stars, or brown dwarfs which are at least 100 AU away from their parent stars.

Planetary-mass objects and rough planets not gravitationally bound to a star can be found through direct imaging as well.

Timeline of discovery

  • In 2004, a group of astronomers used the European Southern Observatory's Very Large Telescope array in Chile to produce an image of 2M1207b, a companion to the brown dwarf 2M1207. The planet is estimated to be several times more massive than Jupiter, and to have an orbital radius greater than 40 AU

  • In September 2008, an object was imaged at a separation of 330 AU from the star 1RXS J160929.1−210524, but it was not until 2010, that it was confirmed to be a companion planet to the star and not just a chance alignment.

Imaging with what

ground-based telescopes

  • Gemini Planet Imager,


  • the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument,

  • Palomar Project 1640,

space telescope


The New Worlds Mission proposes a large occulter in space designed to block the light of nearby stars in order to observe their orbiting planets. This could be used with existing, already planned or new, purpose-built telescopes.

In 2010, a team from NASA's Jet Propulsion Laboratory demonstrated that a vortex coronagraph could enable small scopes to directly image planets, They did this by imaging the previously imaged HR 8799 planets, using just a 1.5 meter-wide portion of the Hale Telescope.

A recent paper Two directly-imaged, wide-orbit giant planets around the young, solar analogue TYC 8998-760-1 Alexander Bohn et al.

  • Even though tens of directly imaged companions have been discovered in the past decades, the number of directly confirmed multi-planet systems is still small.

  • Dynamical analysis of these systems imposes important constraints on the formation mechanisms of these wide-orbit companions.

  • As part of the Young Suns Exoplanet Survey (YSES) the paper reports the detection of a second planetary-mass companion around the 17 Myr-old, solartype star

  • Driven by the installation of extreme adaptive-optics (AO) assisted imagers such as the Gemini Planet Imager and the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE)

  • Pluto, at the edge of our solar system, only lies at 40 au from its host star our Sun, however, this huge exoplanet has a projected physical separation of 320 au and about 6±1 MJup, which is With the previously detected 14 ± 3 MJup companions that are orbiting the primary at 160 au, Making TYC 8998-760-1 the first directly-imaged multi-planet system that is detected around a young star that is like our own Sun.

  • The circular orbits are stable, but that mildly eccentric orbits

  • for either/both components (e > 0.1) are chaotic on Gyr timescales, implying in-situ formation or a very specific ejection by an unseen third companion.

  • Due to the wide separations of the companions, TYC 8998-760-1 is an excellent system for spectroscopic and photometric follow-up with space-based observatories such as JWST

"This discovery is a snapshot of an environment that is very similar to our Solar System, but at a much earlier stage of its evolution," said Alexander Bohn, an astronomer at Leiden University in the Netherlands who led the study


Exoplanets as experiment!!

We might be able to detect dark matter using exoplanets. A massive celestial body interacts with dark matter pretty much only via the gravitational force. So the dark matter will be attracted to a massive object and eventually fall to the core, releasing kinetic heat as it does so, but furthermore here it will get very dense and cause dark matter fusion releasing more heat energy. It might be possible that this heat energy is detectable and can tell us about dark matter.

In a new paper Juri Smirnov, a fellow at The Ohio State University's Center for Cosmology and Astroparticle Physics with Rebecca Leane, a postdoctoral researcher at the SLAC National Accelerator Laboratory at Stanford University. It was published (April 22, 2021) in the journal Physical Review Letters. Outline this new method.

They are excited about Exoplanets for many reasons

  • The exoplanet field is booming we will soon have a very large data set, big data sets mean better science. 4,300 confirmed exoplanets and an additional 5,695 candidates are currently under investigation

  • Neutron stars being very massive would soak up quite a lot of dark matter, but they are very small in size, and an active one would have a radioactive core adding too much noise to the signal. A very large super Jupiter or a brown dwarf however are still massive but also very large diameters meaning you can see them at greater distances, maybe all the way to the centre of the galaxy.

  • Rough exoplanets may be really cool, literally, as they are away from a host star taking away yet another source of the noise.

  • Scientists think that dark matter gets denser toward the centre of galactic disks, and so measuring the DM heating of exoplanets with their position in the galactic disk can tell scientists load about the distribution of dark matter, essentially showing very good evidence of actually finding dark matter too.

  • DMheated exoplanets can be potentially measured when the infrared telescope James Webb Space Telescope (JWST) comes online

  • Might be able to test these models on Jupiter and Saturn too.

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