Check out the companies making headlines in midday trading on Thursday:

E-Trade, Morgan Stanley — Shares of E-Trade surged 21.8% after Morgan Stanley said it will acquire the brokerage for $13 billion. The investment bank will pay $58.74 a share in stock for E-Trade in a deal bringing together $3.1 trillion in client assets. Morgan Stanley shares fell 4.6% on the proposed purchase, the biggest takeover by a U.S. bank since the financial crisis.

Virgin Galactic — Shares of the space tourism slipped 0.2% after a wild day of trading, with heavy volume seeing the stock swing up as more than 13%, past $40 a share, and down as much as 17%. Virgin Galactic has become Wall Street's favorite speculative play, with no public catalyst from the company driving the stock.

Zillow — Shares of the online real estate company gained 16.6% after the company topped revenue estimates in the fourth quarter. The company brought in $944 million, which was above the $814.6 million analysts had been expecting, according to estimates from FactSet. In the same quarter a year earlier, the company reported $365.3 million in revenue. As the online real estate company moves into buying and selling homes, the company said it earned $603 million in its homes segment.

Domino's Pizza — Shares of the pizza chain jumped 25.6% on the back of quarterly results that topped analyst expectations. Domino's reported a fourth-quarter profit of $3.13 per share on revenue of $1.15 billion. Analysts polled by Refinitiv expected a profit of $2.98 per share on revenue of $1.125 billion. Same-store sales — a key metric for restaurants and retailers — grew by 3.4% to beat an analyst estimate of 2.3%.

Copart — Shares of vehicle auction company Copart fell 6.9% after the company missed Wall Street estimates for its second fiscal quarter. The company reported adjusted earnings per share of 65 cents and $575.1 million of revenue. Wall Street analysts expected 66 cents per share and $576.1 million of revenue, according to FactSet. Copart's vehicle sales fell by nearly $3 million compared with the same period last year.

L Brands — Shares of L Brands ticked 3.6% lower on news that private equity firm Sycamore Partners will acquire a 55% share in Victoria's Secret for $525 million. The stock reaction is likely a sign the market was disappointed in the agreement, which puts a value on the lingerie brand of $1.1 billion.

ViacomCBS — Shares of ViacomCBS plunged 17.9% after the newly-merged media company posted an quarterly earnings miss. The company reported adjusted quarterly earnings of 97 cents per share, below the consensus estimate of $1.44 per Refinitiv. Revenue also came in below forecasts. ViacomCBS said the quarter was transitional, and the combination will cut expenses by more than expected.

MEDNAX — Mednax fell 21.2% on Thursday after it issued first-quarter guidance below what Wall Street analysts had been expecting. Pessimism reach an apogee, however, after the neonatal services company said health insurance giant UnitedHealth is canceling its contracts with Mednex in four states. While not too problematic on face value with only $2.5 billion in annual sales at risk, Gordon Haskett analyst Don Bilson notes it could mark the start of a soured relationship. "To have the largest private health insurer in the U.S. make a move like this tells you that MD has some work to do in the area of 'payor relations,'" he wrote.

Synopsys — Shares of the silicon chipmaker fell 6% after reporting first-quarter results, with Synopsys forecast for revenue next quarter coming slightly below what analysts surveyed by FactSet expected. However, the company's first-quarter earnings and revenue were both stronger than Wall Street expected, as well as its 2020 forecast for revenue.

Paycom Software — Shares of Paycom fell 4.4% following an announcement that fellow workplace-software companies Ultimate Software and Kronos had agreed to merge. The combined firm will be worth roughly $22 billion, according to the companies. Paycom, which joined the S&P 500 last month, is still up more than 17% for the year.

Six Flags Entertainment — Shares of Six Flags tanked 16.1% after the theme park operator posted a quarterly loss. It also gave a weaker-than-expected earnings outlook for 2020, while cutting its dividend by nearly 70%. Six Flags also announced that its chief financial officer plans to retire at the end of August.

Climarex Energy — Shares rose 6.5% after revenue in the fourth quarter topped estimates. The company earned $657.2 million, while analysts polled by FactSet had been expecting $629.8 million. Excluding items the company earned $1.18 per share, which was short of the $1.24 analysts had been expecting.

— CNBC's Maggie Fitzgerald, Yun Li, Pippa Stevens, Jesse Pound, Fred Imbert and Tom Franck contributed to this report.

Subscribe to CNBC PRO for exclusive insights and analysis, and live business day programming from around the world.