Recent research

Here are some brief descriptions of research I have been working on, together with links to the relevant published articles or preprints.

The planets with an electric heater turned to high

posted Nov 4, 2018, 9:19 AM by Jeremy Drake   [ updated Nov 4, 2018, 9:22 AM ]

We now know that planets are common around M dwarfs, and the 100 billion or so M dwarfs in our own Galaxy mean the chance of there being "habitable" worlds with liquid water on their surface should be extremely high.  But liquid water is not the only consideration for habitability. The magnetic fields of M dwarfs and their associated energetic radiation - both in the form of a plasma "wind" and UV to X-ray emission - are generally much stronger in comparison to their visible light than for solar-like stars. As a consequence, close-in planets around M dwarfs endure a distinctly more hostile space environment than the Earth does ,and an environment that might strip planet's atmosphere and surface water. 

The wind from a star like the Sun - including M dwarfs - is not just plasma; it also carries with it magnetic field that originates within the star in a magnetic dynamo driven by the star's rotation. When encountering a planet, the moving magnetic field and stellar wind plasma generate electric currents within the planetary ionosphere called "Birkeland currents", after Norwegian explorer and physicist Kristian Birkeland who first proposed them.  The ionosphere of a planet is the very outer atmosphere where ionization by the Sun's UV, EUV and X-ray light creates a significant number of ions and free electrons. In the Earth's case, the ionosphere is the region between 60 and 1000 km or so in altitude.  The currents that flow heat up the ionosphere in a process called "Joule heating", just like in an electric heater.  Enough energy is injected by Joule heating to expand and make a significant difference to the extent of the Earth's very outer atmosphere.

University of Massachusetts researcher, Prof. Ofer Cohen, lead a study published in the Astrophysical Journal Letters to estimate the heat injected into the the ionospheres of the planets around the M dwarf TRAPPIST-1.  The TRAPPIST-1 planets experience stellar wind conditions up to 100,000 times stronger than Earth, and Cohen's team found that, if the TRAPPIST-1 planets have managed to retain an atmosphere, the Joule heating rate is commensurately elevated by enormous amounts.  The net effect will be to increase the planet's atmospheric loss rate, likely further jeopardising their capability to harbour and sustain life over billion year timescales.

Finding Life in... Outer Space?!

posted Mar 23, 2018, 10:56 AM by Jeremy Drake

Back in 2013, I proposed a three part film series to the Smithsonian Channel on "Life in the Cosmos" - an attempt to look into all the astrophysics involved in the origin and evolution of life.  They liked the proposal and looked for partners and funding. A couple of false starts and a year or two later and the scope was down to two films. We ended up making one that was filmed last year. The second morphed into an edit of a BBC-commissioned film featuring the great Stephen Hawking - I think his last film appearance.  Ok, maybe just cut to the PR blurb...

“I wanted to look at a different slant to the story of the discovery of exoplanets,” said Drake, “including more of the astrophysics that goes into it and the complex science rationale behind the possibility of life developing somewhere other than Earth.” 

“Finding Life in Outer Space” travels across incredible tracts of space and time exploring the greatest mystery of the universe - why life exists. The show follows the scientists responsible for some of the biggest breakthroughs in understanding the origins of life and uncovers how cutting-edge observations and experiments are transforming the view of how the universe works. 

Hydrothermal geyser pools in the Atacama Desert in Chile reveal secrets about the first steps life took on the primeval Earth, and high in the Arctic Circle, the northernmost science facility on the planet keeps an eye on the magnetic shield that protects humans from devastation by the solar wind. 

The film explores U.S laboratories where the secrets of astrochemistry are recreated, and looks to England where they are closing the gap between chemistry and biology by simulating conditions on Earth four billion years ago. Telling a story of creation on an amazing scale and with incredible consequence, “Finding Life in Outer Space” shows that science may be pointing to one fantastic conclusion - we are not alone.

X-rays vs Protons: Dawn of Creation

posted Mar 23, 2018, 10:25 AM by Jeremy Drake   [ updated Mar 23, 2018, 10:56 AM ]

Planets are formed in the remnant "protoplanetary disk" of gas surrounding a newborn star. The process is one of hierarchical merging, of dust grains to form planetesimals, and planetesimals to form protoplanets that are then large enough to accrete material from the disk by their own gravity. Planets also interact with the gas in the disk, which can pull it closer or push it further away from its host star. Natal planetary characteristics and their resulting orbits then depend critically on the properties of the disk and how long it hangs around.

If it were not for energetic radiation from the host star, the gas disk would hang around for many millions of years. Instead, X-rays and fast protons heat the very outer layers and drive off mass in a disk wind.  They also weakly ionize the gas, whose charged particles interact through electric and magnetic forces and make the gas viscous. The viscous gas slows itself down in its orbit and gradually spirals into the central star - see the posting on an observation of this at work. Within 10 million years the gas disk is gone.

Stellar X-rays had long been thought the principle driver of these processes, until an estimate of energetic proton fluxes suggested instead that they were responsible. This estimate assumed protons travel in straight lines from their acceleration sites in the star's flares and coronal mass ejections. But charged particles follow curved magnetic field lines, and young stars have strong magnetic fields - several hundred times stronger that the Sun's.  We simulated the trajectories of energetic protons within the magnetosphere and wind environment of a young "T Tauri" star.  We found most of the protons get trapped by the strong stellar magnetic field.  Those that escape follow the magnetic field in particular trajectories, hitting the protoplanetary disk in specific places in a mottled pattern. They cannot dominate the global disk ionization in this way, but disk models will need to be extended to understand the effects of the strong, localized bombardment that can dominate the ionization in specific places where the magnetic field intercepts the disk. This work was lead by SAO visiting scientist Federico Fraschetti, and was published in the 2018 February 1 edition of the Astrophysical Journal.

The Ring of Darkness

posted Feb 26, 2018, 10:11 AM by Jeremy Drake

The speed with which stars orbit around the centre of a galaxy - the ``rotation curve'' - tell us about the distribution of mass within the galaxy.  One of the most remarkable discoveries made in astronomy employed this technique of examining the orbital speeds of stars in galaxies that are viewed nearly edge-on.  The data indicated that most of the mass of a typical galaxy lies in an extended halo beyond the visible stars and gas .  This otherwise unseen mass was dubbed ``dark matter".   

The importance of stellar kinematics for understanding galaxies gives the impression that our own Milky Way rotation curve would be well-defined and comprehensively observed.  Instead, the kinematics of the Galaxy outside the Solar Circle - the orbit of the Sun about the Galactic centre -  is quite poorly known. University of Hertfordshire PhD student Amy Harris set out to show that stars slightly more massive than the Sun, of spectral type A to F, can be uniquely exploited to remedy the situation.  These types of star can be efficiently selected from photometric surveys.  Being relatively bright, they can be seen to large distances, and being relatively young they still reflect the kinematics of Galactic rotation.  

Amy used Multiple Mirror Telescope observations of a sample of about 800 A/F stars to probe a narrow 1 degree sightline across our Galaxy and found a surprising increase in orbital speed with increasing distance from the Galactic centre. The results support earlier suggestions that the Milky Way has ring of dark matter lying between 13 and 18 kpc from the centre. These are just the first pilot study results and Amy's method is now set to be deployed to test these findings in much more extensive upcoming surveys.  Amy's work was published in the 2018 January 5 edition of the Monthly Notices of the Royal Astronomical Society.

A star of the beat generation

posted Dec 2, 2017, 1:50 PM by Jeremy Drake

Stars like the Sun generate copious magnetic fields that induce all sorts of interesting behaviour at the stellar surface.  Sunspots betraying bundles of emerging magnetic field, and the diffuse white halo of light scattered off the magnetically-driven solar wind seen around the Sun during a total eclipse, are good examples of this. The magnetic field is generated by the combination of the Sun's rotation and the convection that characterises the outer 30% or so of its radius. The magnetic field is always changing and goes through a cycle in which the polarity of the fieldthe "north" and "south" of the magnetreverses and changes back again over a period of about 22 years.  Similar cyclic magnetic behaviour is seen in about 60% of Sun-like stars.

Iota Horologii is a star like the Sun but much younger, with an age of "only" about 600 million years. It came to prominence when found to host a planet with a mass of 2.5 Jupiters in an orbit similar to that of Earth.  ι Hor rotates four times faster than the Sun because it has not had time to lose as much angular momentum through its wind. This faster rotation drives a stronger magnetic dynamo. Earlier studies had found a much shorter magnetic cycle than the Sun's with a period of only 1.6 yearsin fact the shortest known to date. As part of a much wider investigation of this intriguing exoplanet host, SAO Postdoctoral Scholar Julian Alvarado-Goméz studied the light from ionized calcium atoms in the star's upper atmosphere, or "chromosphere."  The chromosphere is heated by energy dissipated from the surface magnetic field and so can be used to trace the magnetic cycle. In a paper published in the 2017 October 10 edition of the Monthly Notices of the Royal Astronomical Society, Alvarado-Goméz and colleagues find the cycle is actually two cycles, with periods of 1.4 and 2 years, superimposed and "beating" against each other to make an apparent 1.6 year average. Still to be done is to understand exactly how and why magnetic cycles on the Sun and stars arise at all.

The icing on the snake

posted Nov 9, 2017, 7:37 AM by Jeremy Drake   [ updated Nov 9, 2017, 7:37 AM ]

Planets are born in the residual gas and dust from the star formation process that is left orbiting a nascent star in the form of a disk. The closest example of such a planet hatchery is in the constellation of Hydra, the water snake. The TW Hydrae system, whose Atacama Large Millimeter Array image famously shows a disk face-on to us carved out with dark lanes that could possibly be gaps swept out by newly-born planets.  The gas disk lasts for a few million years, eventually being dissipated by evaporation or loss to a disk wind, or by spiraling onto the parent star under gravity.

The inner edge of the disk is truncated at a few stellar radii by the star's strong  magnetic field.  Gas then follows the magnetic field lines, accreting onto the star in narrow columns down the magnetic poles. In free-fall, velocities reach up to 500 km per second and most of this energy of motion is converted into heating the gas to 2-3 million degreeshot enough to emit at X-ray wavelengthsin a shock as it hits the stellar surface. The shocked gas is slowed by a factor of about 4, so just before merging into the stellar surface it can still be moving at about 100 km per second.  Italian astrophysicist Costanza Argiroffi, from the Palermo Astronomical Observatory in Sicily, lead a study published in the November 2017 edition of Astronomy & Astrophysics to try and detect this motion of the shocked gas accreting onto TW Hydrae by its Doppler shift using the spectrometers onboard the Chandra X-ray Observatory. By combining many observations of the star and carefully comparing the spectrum with older stars long-bereft of accreting gas, the Doppler shift was detected. But the velocity was much smaller than expected - "only" 40 km per second.  This indicates that the accretion streams that provide the last 1% or so of the star's final massthe icing on the cakelie at an angle to our line-of-sight. The dominant magnetic pole of TW Hydrae is then not at the geographical stellar pole, but instead closer to the equator at a latitude of 10-30 degrees.

Monster CME from the Demon Star

posted Oct 30, 2017, 9:10 PM by Jeremy Drake   [ updated Nov 1, 2017, 8:06 AM ]

The Arabic name "Algol", of the eclipsing close binary star sometimes known as the Demon Star, derives from the "head of the ghoul"in Greek tradition the head of the venomous snake-haired Medusa severed by the demigod fellow Perseus. It comprises a B8 dwarf and a K0 subgiant in a 2.86 day orbit.  When the fainter K star passes in front of the brighter B star, Algol dims significantlybehaviour thought of as somewhat spooky long before its binary nature was known.  The stars are tidally-locked, and both rotate with the same period as their orbit, much like the Moon is tidally-locked to its orbit about the Earth.  The consequent rapid rotation of the K star, in concert with its outer convection zone, engenders vigorous and violent magnetic activity.  The hot B8 dwarf is instead bereft of an outer convection zone and appears magnetically quiescent.  This is handy because when the B star passes in front of the K star the eclipse signature in X-rays can be used to infer the spatial structure of the K star's X-ray emitting corona.

Back in late August of 1997, the joint Italian-Dutch BeppoSAX X-ray satellite caught Medusa's head having a bad hair day. An immensely energetic magnetic flare was observed that lasted  for nearly three days, and released a total X-ray energy of 1037 ergequivalent to the entire visible light output of the Sun for almost an hour and a million times more energetic than the largest solar flares. The flare was eclipsed by the B star, enabling the scientists who first studied it to estimate its size and location. Part of the X-ray signal from the flare was also observed to suddenly dim, then recover.  It was suggested that this dimming was caused by a coronal mass ejection like those that commonly accompany large solar flares.  SAO Postdoctoral Scholar Sofia Moschou took up this idea and analysed the ejection using a geometrical CME description developed for solar CMEs called the "ice cream cone" model.  She found the event had a mass of two to twenty thousand trillion tonnes, similar to that of the fifth largest asteroid in the solar system, Interamnia, and moved at a speed of about 3 million kph.  This work, lead by Dr. Moschou, is accepted for publication in the Astrophysical Journal.


posted Aug 28, 2017, 9:19 PM by Jeremy Drake   [ updated Aug 28, 2017, 9:26 PM ]

It was the first time I saw the solar corona with my own eyes.  Having worked in the field of stellar coronae for more than two decades, the eclipse of 2017 August 21 was quite special to me. 
In 1991, Wilton Barnhardt—a friend from Oxford days—and I travelled to Mexico on an extensive Kerouac-esque (well, perhaps at times...), road trip designed to take in the 1991 July 11 eclipse. The southern end of the Baja Peninsular, close to La Paz, was predicted to be the best spot to see it from. But it's quite a long drive down—900 miles or so from Tijuana—and we would have missed some places in northern Mexico we wanted to stop at. While Baja's Valle De Los Cirios was appealing, the ferry to the mainland would have taken an additional day, and considerable cash that was not in great supply. So, we opted for Mazatlan on the mainland that also made a good stopping point for continuing the trip.
The morning was clear, with blue skies and a smattering of clouds toward the horizon.  The early partial phase was nicely visible, starting around noon, through my number 14 welders' glass held more precariously than I'd like to admit by a complex but not entirely convincing web of elastic bands to my binoculars.  But, as noon turned to afternoon, we watched with mounting dismay, like Macbeth beholding the rise of Birnam Wood to Dunsinane, as clouds rolled in to blot out the remainder of the eclipse.  Totality under clouds is horribly disappointing—it got darker, street lights came on, and then the light gradually returned again. That's about it.  We hit the road and moved on, me as the astronomer feeling a bit self-conscious about having earlier talked up the damp squib eclipse so much to my friend.  I would not get a good chance to see another one until last week.

Of course, it's an utterly fatuous exercise to try and photograph the eclipse—hundreds, thousands, of eclipse experts with telescopes and fantastic professional-grade gear will do an infinitely superior job to a naive tourist with a consumer-level camera, and will plaster their beautiful images all over the web for anyone to get hold of.  What's the point of me doing it? I suppose its a bit like wearing that old "Beckham" number 7 shirt while playing football (soccer, if you must!) rather poorly at lunchtimes, before I did my knee in.  Or muddling through Agustin Barios' La Catedral on classical guitar, making a hash of it in the same places every single time, and the other side of the Universe from the brilliance of Ana Vidovic's rendition that is so dazzlingly beautiful it brings tears to my eyes.  The pictures are indeed a bit rubbish in the grand scheme of eclipse photography, and at the time of writing I've not had time to do anything but download a couple of them straight from the camera. But they are mine, and I took them at that place and time when I actually got to see the solar corona.

Gone with the wind

posted Jul 25, 2017, 5:21 AM by Jeremy Drake   [ updated Nov 6, 2017, 8:16 AM ]

The discovery of a planet orbiting our nearest star, Proxima Centuri, is a jewel in the crown of exoplanetary science. Further gilded by its location in the "habitable" zone, where an Earth-like planet would not freeze or boil its oceans, Proxima b represents a chance that the nearest exoplanet to us could actually harbour life. 

Surface liquid water requires the presence of a substantial atmosphere - without atmospheric pressure, water simply boils to the gaseous state. Close proximity to Proxima poses problems in this regard: as noted previously, the intense stellar wind where Proxima b resides could strip away a planetary atmosphere.  A strong planetary magnetic field might provide some protection, but atmospheric escape can still occur through the magnetic poles.  This "polar wind" occurs on Earth, but is not strong enough to endanger our atmosphere.  It occurs because air molecules at high altitude get ionized by solar extreme ultraviolet and X-ray radiation, and the light electrons liberated in the process can rise to sufficient altitudes to begin to escape Earth's gravity.  The rise of electrons generates an electric field that pulls the positively charged heavier ions upward, essentially resulting in a flow into space.  

Proxima b experiences ionizing radiation levels hundreds of times higher than the Earth, and we expected its polar wind to be commensurately stronger.  In a study lead by Katie Garcia-Sage of the NASA Goddard Space Flight Center and the Catholic University of America, featured in a NASA release, a Forbes article, and published in the July 20th edition of the Astrophysical Journal Letters, we used supercomputer models to compute the ionospheric outflow that the Earth would experience were it in the location of Proxima b. A best guess for the thermospheric temperature suggests a polar wind escape rate sufficient to remove an Earth atmosphere in a few hundred million years - a short timescale compared with the planet's likely age of nearly 5 billion years.  While the atmosphere could plausibly be replenished by geological processes, such as volcanic activity, the loss rates we estimate are conservative and do not include other erosion effects from the stellar wind and coronal mass ejections.  In all likelihood, Proxima b is bare, devoid of an atmosphere that has gone with the wind.

Check the space weather before packing

posted Jul 17, 2017, 7:36 AM by Jeremy Drake   [ updated Jul 17, 2017, 7:36 AM ]

The remarkable TRAPPIST-1 system is now know to host at least seven planets, three of which reside in the so-called temperate or "habitable zone", where surface temperatures should in principle be able to sustain water in liquid form.  But, as we have argued in earlier postings, there is a big difference between a planet being in the habitable zone and actually being habitable. Liquid water, in particular, can only exist within a relatively dense atmosphere. 

TRAPPIST-1 is an "utracool" red dwarf - a tiny star one tenth the size of the Sun with a surface temperature of about 2600 degrees.  It is so faint that only planets that orbit really close in can have temperatures warm enough for liquid water. The problem for the TRAPPIST-1 planets is that their star is not so weak at ultraviolet to X-ray wavelengths and also in regard to its magnetic field and wind. TRAPPIST-1 rotates once every 3.3 days, and this rapid rotation has built up its surface magnetic field to an immense strength of 100 times our Sun's.  Despite the star being so small, this magnetic field drives a stellar wind that ends up being of a similar strength to the solar wind.  Because the wind strength experienced by a planet decreases with the square of its distance from its parent star, the TRAPPIST-1 planets in their close-in habitable zone get blasted by a wind up to ten thousand times stronger than the Earth experiences.

On Earth, we are protected from most of the ravages of the solar wind by the Earth's magnetic field. However, any magnetic fields the TRAPPIST-1 planets have will be largely overwhelmed by the magnetism of their star.  The star-planet magnetic fields can directly connect over much of each planet, allowing the full force of the stellar wind to stream straight onto their surfaces.  The concept of atmospheric protection by a planetary magnetic field does not hold here. These conditions will likely strip off any atmospheres - and liquid water - the TRAPPIST-1 planets might have had on timescales of a hundred million years or so. Most habitable zone planets around similar low mass M dwarfs are likely to end up dry and barren - uninhabitable in the habitable zone. 

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