This artist's concept depicts select planetary discoveries made to date by NASA's Kepler space telescope in this image released May 10, 2016.  Courtesy W. Stenzel/NASA/Handout via REUTERS    ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. EDITORIAL USE ONLY

W. Stenzel/NASA/Reuters

This artist’s concept depicts select planetary discoveries made to date by NASA’s Kepler space telescope

BILLINGSHURST, ENGLAND - MAY 11:  An auction house employee attaches the leg to the skeleton of a Eohippus, during the press preview of a forthcoming sale at Summers Place Auction House on May 11, 2016 in Billingshurst, England. The piece is expected to fetch between £5,000 - £8,000 GBP when it goes up for sale. Summers Place Auction house will sell the museum collection of the Emmen Zoo, an internationally acclaimed zoo in northern Holland, at their sale on June 7th, 2016 in Billinghurst, Surrey.  (Photo by Dan Kitwood/Getty Images)

Dan Kitwood/Getty

An auction house employee attaches the leg to the skeleton of an Eohippus, during the press preview of a forthcoming sale at Summers Place Auction House in Billingshurst, England

A participant in the ''Bread Procession of the Saint'' takes part in a ceremony in honor of Domingo de La Calzada Saint (1019-1109), in Santo Domingo de La Calzada, northern Spain, Wednesday, May 11, 2016. Every year during spring season, ''Las Doncellas'' (White Virgins), hold a basket on their head covered with white cloth while they walk along this old village in honor of the saint.

Alvaro Barrientos/AP

A participant in the ”Bread Procession of the Saint” takes part in a ceremony in honour of Domingo de La Calzada Saint (1019-1109), in Santo Domingo de La Calzada, Spain. Every year during spring season, ”Las Doncellas” (White Virgins), hold a basket on their head covered with white cloth while they walk along this old village in honor of the saint 

U.S. servicemen wait for an opening ceremony of U.S. led joint military exercise "Noble Partner 2016" in Vaziani, Georgia, May 11, 2016. REUTERS/David Mdzinarishvili

David Mdzinarishvili/Reuters

U.S. servicemen wait for an opening ceremony of U.S. led joint military exercise “Noble Partner 2016″ in Vaziani, Georgia 

Nigerian President Muhammadu Buhari list...Nigerian President Muhammadu Buhari listens at the start of a conference to tackle corruption at the Commonwealth Secretariat in London on May 11, 2016. / AFP PHOTO / LEON NEALLEON NEAL/AFP/Getty Images

Leon Neal/AFP

Nigerian President Muhammadu Buhari listens at the start of a conference to tackle corruption at the Commonwealth Secretariat in London

Right-wing organisation Hindu Sena perform Hindu fire rituals in support of Donald Trump in New Delhi...epa05298860 Indian activists from the right-wing organisation Hindu Sena perform Hindu fire rituals in support of US Republicans presidential candidate Donald Trump in New Delhi, India, 11 May 2016. Right-wing organisation Hindu Sena activists prayed to the Hindu gods and performed rituals to ensure the victory of Donald Trump in US Presidential elections and urged the Indian origin people living in USA to vote and support him.  EPA/RAJAT GUPTA

Rajat Gupta/EPA

Indian activists from the right-wing organisation Hindu Sena perform Hindu fire rituals in support of US Republicans presidential candidate Donald Trump in New Delhi, India 

A boy sit on a tank on display at the Armored Corps memorial, before a ceremony marking the annual Memorial Day for soldiers and civilians killed in more than a century of conflict between Jews and Arabs, in Latrun near Jerusalem, Israel, Wednesday, May 11, 2016. Israel came to a standstill on Wednesday as sirens wailed across the country on its annual Memorial Day for fallen soldiers and victims of terrorism. (AP Photo/Ariel Schalit)

Ariel Schalit/AP

A boy sit on a tank on display at the Armored Corps memorial, before a ceremony marking the annual Memorial Day for soldiers and civilians killed in more than a century of conflict between Jews and Arabs, in Latrun near Jerusalem, Israel

Presidential Candidate Hillary Clinton Holds Kentucky Campaign Rally...An attendee wears campaign buttons before the start of an event for Hillary Clinton, former Secretary of State and 2016 Democratic presidential candidate, not pictured, in Louisville, Kentucky, U.S., on Tuesday, May 10, 2016. Clinton said she sees "a great role" for Bernie Sanders and his supporters in a "unified party," even as she said she welcomed Republicans who are not supporting presumptive nominee Donald Trump. Photographer: Luke Sharrett/Bloomberg

Luke Sharrett/Bloomberg

An attendee wears campaign buttons before the start of an event for Hillary Clinton, former Secretary of State and 2016 Democratic presidential candidate, in Louisville, Kentucky 

Members of the Landless Workers Movement (MST) take part in a protest against the impeachment of Brazil's President Dilma Rousseff in Brasilia, Brazil, May 10, 2016. REUTERS/Ueslei Marcelino

Ueslei Marcelino/Reuters

Members of the Landless Workers Movement (MST) take part in a protest against the impeachment of Brazil’s President Dilma Rousseff in Brasilia, Brazil 

Britain's opposition Labour Party leader Jeremy Corbyn and shadow Minister for Young People, Gloria De Piero (L) stand in front of the "Labour In For Britain" campaign bus at a launch in central London, on May 10, 2016. The referendum campaign was picking up pace again after regional and local elections last Thursday which saw Labour's Sadiq Khan elected as London's new mayor, and pro-independence nationalists returned to power in Scotland, albeit without a majority. Main opposition Labour leader Jeremy Corbyn was now being urged to get more involved in the EU referendum campaign.  / AFP PHOTO / LEON NEALLEON NEAL/AFP/Getty Images

Leon Neal/AFP

Britain’s opposition Labour Party leader Jeremy Corbyn and shadow Minister for Young People, Gloria De Piero stand in front of the “Labour In For Britain” campaign bus at a launch in central London 

A bee collects pollen from a bluebell fl...A bee collects pollen from a bluebell flower at the Herrenhaeuser Gaerten park in Hanover, central Germany, on May 9, 2016. / AFP PHOTO / dpa / Sebastian Gollnow / Germany OUTSEBASTIAN GOLLNOW/AFP/Getty Images

Sebastian Gollnow/AFP

A bee collects pollen from a bluebell flower at the Herrenhaeuser Gaerten park in Hanover, Germany 

TOPSHOT - Palestinian children play amid...TOPSHOT - Palestinian children play amidst wrecked cars in an impoverished area in the southern Gaza Strip city of Khan Yunis, on May 9, 2016.  / AFP PHOTO / THOMAS COEXTHOMAS COEX/AFP/Getty Images

Thomas Coext/AFP

Palestinian children play amidst wrecked cars in an impoverished area in the southern Gaza Strip city of Khan Yunis 

Wearing traditional military uniform, a member of a South Korean military honor guard, salutes before a welcoming honor guard ceremony for U.S. Gen. Vincent Brooks, commander of United Nations Command, US Forces Korea and Combined Forces Command at South Koreaπs Joint Chiefs of Staff in Seoul, South Korea, Tuesday, May 10, 2016. (AP Photo/Lee Jin-man)

Lee Jin-man/AP

Wearing traditional military uniform, a member of a South Korean military honour guard, salutes before a welcoming honour guard ceremony for U.S. Gen. Vincent Brooks, commander of United Nations Command, US Forces Korea and Combined Forces Command at South Korea’s Joint Chiefs of Staff in Seoul, South Korea

Vehicles stop on the side of a road as a...Vehicles stop on the side of a road as a tornado rips through a residential area after touching down south of Wynnewood, Oklahoma on May 09, 2016.  The tornado touched down quickly and destroyed an unknown number of structures before a series of other twisters riddled the area. One person is confirmed dead. == RESTRICTED TO EDITORIAL USE  / MANDATORY CREDIT:  "AFP PHOTO / HANDOUT / Josh EDELSON" / NO MARKETING / NO ADVERTISING CAMPAIGNS /  DISTRIBUTED AS A SERVICE TO CLIENTS  / GETTY OUT ==

Josh Edelson/AFP

Vehicles stop on the side of a road as a tornado rips through a residential area after touching down south of Wynnewood, Oklahoma. The tornado touched down quickly and destroyed an unknown number of structures before a series of other twisters riddled the area. One person is confirmed dead. 

Presidential Candidate Bernie Sanders Holds Atlantic City Campaign Rally...An attendee with a campaign sticker on her phone takes video as Senator Bernie Sanders, an independent from Vermont and 2016 Democratic presidential candidate, not pictured, speaks during a rally at Boardwalk Hall in Atlantic City, New Jersey, U.S., on Monday, May 9, 2016. Atlantic City, the struggling casino hub weeks away from bankruptcy absent state action that has been hit hard by the decline of its dominant industry, is a fitting platform for Sanders populist message on how corporations reap profits while urban areas are neglected. Photographer: Andrew Harrer/Bloomberg

Andrew Harrer/Bloomberg

An attendee with a campaign sticker on her phone takes video as Senator Bernie Sanders as he speaks during a rally at Boardwalk Hall in Atlantic City, New Jersey

TOPSHOT - A man jumps into the water of ...TOPSHOT - A man jumps into the water of the Freibad Kiebitzberge public open air swimming pool in Kleinmachnow near Potsdam, northeastern Germany, on May 9, 2016. / AFP PHOTO / dpa / Ralf Hirschberger / Germany OUTRALF HIRSCHBERGER/AFP/Getty Images

Ralf Hirschberger/AFP

A man jumps into the water in a swimming pool in Kleinmachnow, Germany

Indian childred cool themselves at a fou...Indian childred cool themselves at a fountain near India Gate in New Delhi on May 9, 2016 India is officially in the grip of its worst water crisis in years, with the government saying that about 330 million people, or a quarter of the population, are suffering from drought after the last two monsoons failed. / AFP PHOTO / MONEY SHARMAMONEY SHARMA/AFP/Getty Images

Money Sharma/AFP

Indian children cool themselves at a fountain near India Gate in New Delhi

Europe Day in Rome...epa05296191 People take part in a ceremony on the occasion of the Europe Day in Rome, Italy, 09 May 2016. The Europe Day is aimed to celebrate peace and unity in the continent.  EPA/MASSIMO PERCOSSI

Massimo Percossi/EPA

People take part in a ceremony on the occasion of the Europe Day in Rome. Europe Day is aimed to celebrate peace and unity in the continent 

Two Palestinian schoolboys walk past a g...Two Palestinian schoolboys walk past a graffiti painted on a wall of the United Nations school of Beit Hanun, in the northern Gaza Strip, on May 9, 2016.  / AFP PHOTO / THOMAS COEXTHOMAS COEX/AFP/Getty Images

Thomas Coex/AFP

Two Palestinian schoolboys walk past a graffiti painted on a wall of the United Nations school of Beit Hanun, in the northern Gaza Strip

Swimming- European Aquatics Elite Championships- Solo free preliminary synchronized swimming- London...Swimming- European Aquatics Championships- London, Britain, 9/5/2016. Anna Voloshyna of Ukraine competes during solo free preliminary synchro event.       REUTERS/Andrew Boyers

Andrew Boyers/Reuters

Anna Voloshyna of Ukraine competes during solo free preliminary synchro event at the European Aquatics Championships in London